Lump Under the Skin: What Causes It? A Dermatologist Explains How to Tell an Epidermoid Cyst from a Lipoma, Which Lumps Are Dangerous, and When Surgery Is Needed
You feel a bump under the skin on your neck, back, arm or behind your ear, and your heart sinks as you wonder whether it is something serious. This is one of the most common worries in the clinic room. The good news is that most lumps under the skin are benign growths. But many people are confused because a "lump under the skin" is not one single condition, and each type has a completely different nature, risk and treatment.
In this article, a dermatologist explains what causes lumps under the skin, how to tell an epidermoid cyst from a fatty lump (lipoma) and other lumps, why squeezing or picking at them yourself makes things worse, which lumps are warning signs that need prompt medical attention, and when removal is advisable.
What causes a lump under the skin?
"Lump under the skin" is the term patients use for anything they can feel beneath the surface, but medically these lumps arise from different mechanisms. Understanding the origin helps identify the type and assess the risk. Common causes include:
Blockage and build-up of keratin in a sac under the skin: this forms a cyst, a walled sac that contains material inside.
Abnormal growth of fat cells: this forms a fatty lump, or lipoma, a benign tumor of fat cells.
Bacterial infection: this causes an abscess, or pus-filled lump, which is inflamed, red and painful.
Swollen lymph nodes: caused by infection or inflammation nearby, felt as lumps in the neck, armpit or groin.
Other growths: both benign ones and ones that need monitoring, which is why lumps with certain features need to be examined by a doctor.
How many types of lumps are there, and how do you tell them apart?
This is the heart of the article, because guessing the wrong type means the wrong care from the start. A dermatologist tells the common lumps apart by how they feel and where they are located, as follows.
1. Epidermoid cyst — the lump often wrongly called a "fatty cyst"
This is the most common lump under the skin. Its typical features are:
A round, smooth lump that moves slightly and sits close to the skin.
It often has a small central opening (central punctum), seen as a dark dot like a blocked pimple.
When inflamed or squeezed, it may release a thick, yellow-white, foul-smelling material (keratin), which is not "fat" as many believe.
Commonly found on the face, neck, back and trunk.
A point doctors would like you to understand differently: in Thailand (and on many websites) this lump is often called a "sebaceous cyst," but in fact most of these do not arise from the sebaceous glands. Dermatology bodies have stopped using the term sebaceous cyst because these lumps come from epidermal skin cells trapped under the skin that keep producing keratin, not from fat. Using the correct name helps explain why "cutting down on fatty food" or "squeezing the fat out" does not help.
2. Pilar cyst (scalp cyst) — a relative of the epidermoid cyst
It arises from cells of the hair root, so it is found mostly on the scalp.
It feels firmer and rolls around more easily than an ordinary epidermoid cyst.
It usually has no central opening, often occurs as several lumps, and tends to run in families.
3. Fatty lump, or lipoma — a tumor of fat cells
This is the next most common lump under the skin and the one people most often confuse with a cyst. Its typical features are:
Soft and rubbery, not painful when pressed, and clearly movable or rollable under the skin.
No central opening and no thick material coming out (unlike a cyst).
Grows very slowly, usually on the neck, shoulders, back, arms and thighs.
It is a benign tumor of fat cells, not cancer, and in typical cases does not turn into cancer.
4. Abscess, or pus-filled lump — a lump that is "inflamed," not just a "lump"
Red, warm, swollen and painful, and growing quickly within a few days.
There may be a fever, and the center feels soft because of the pus.
It is caused by a bacterial infection, unlike cysts and lipomas, which are normally painless (a cyst becomes painful only when it is inflamed or infected).
Important point: if your lump is red, warm, painful and growing fast, always think of infection first and see a doctor. Do not squeeze it yourself.
5. Steatocystoma multiplex
These are many subcutaneous cysts at the same time, filled with clear, odorless oil.
They usually appear on the chest, trunk and upper arms, from the teenage years to early adulthood.
Unlike an ordinary epidermoid cyst, they appear in groups and are genuinely related to the sebaceous glands.
6. Swollen lymph node
Felt as a round or oval lump that is movable in the neck, armpit or groin.
If it is swollen from an infection, it is tender and shrinks when the infection clears, but a lump that is hard, firmly fixed, painless and keeps growing needs to be checked promptly.
Quick comparison — how does an epidermoid cyst differ from a fatty lump (lipoma)?
The question people ask most is "Is my lump a cyst or a lipoma?" A simple way to tell them apart is as follows:
How it feels: a cyst is usually firm and attached to the skin, while a lipoma is soft, rubbery and clearly rolls.
Central opening: a cyst often has a dark dot in the middle, a lipoma does not.
Material that comes out: a cyst may release thick, yellow-white, foul-smelling material when inflamed, a lipoma releases nothing.
Infection and inflammation: a cyst can become inflamed with pus, a lipoma almost never becomes inflamed.
Common locations: cysts on the face, neck, back and scalp, lipomas on the neck, shoulders, back, arms and thighs.
Although you can make a rough distinction, some lumps look in-between. A definite diagnosis requires examination by a doctor, and sometimes an ultrasound or a biopsy.
Why do lumps not go away, and come back, when you squeeze, pick or puncture them yourself?
This is a misunderstanding that brings many patients in with chronic inflamed wounds. Many people try to squeeze a cyst like a pimple, thinking that if the "fat" inside is removed the lump will disappear. In reality:
A cyst has a sac wall around it. Squeezing out the contents helps only temporarily, but the sac wall remains, so it keeps producing new material and the lump always returns.
Squeezing or puncturing it yourself can rupture the sac wall under the skin, causing severe inflammation, swelling, redness and pain, and making secondary infection easy.
Once a cyst has been inflamed and ruptured, scar tissue and adhesions form, making later surgical removal harder and the chance of recurrence higher.
Key lesson: a permanent cure for a cyst means removing the entire sac wall, not just the material inside. A lipoma also cannot be squeezed down, because it is solid fatty tissue, not a sac of fluid.
Why antibiotics alone do not cure it completely
Many patients take antibiotics, the lump shrinks and they think it is cured, but then it comes back. The reasons are:
Antibiotics help only when the cyst has a secondary infection or inflammation. They reduce inflammation and pus but do not get rid of the cyst sac itself.
Once the inflammation settles, the sac is still there, so the lump remains and can become inflamed again.
The permanent treatment for a problematic cyst is complete surgical removal of the sac while the lump is not inflamed.
Which lumps under the skin are dangerous — warning signs that need prompt medical attention
Most lumps under the skin are not serious, but some features are regarded by doctors as warning signs and need examination to rule out growths that need monitoring. See a doctor if a lump has the following features:
Very hard, firmly fixed and immovable, as if attached to deeper tissue.
Unusually fast growth within a few weeks to months.
Larger than 5 centimeters or continually growing without stopping.
Painful without any infection, or accompanied by numbness.
Changes in the skin over the lump, such as an ulcer, bleeding or abnormal color.
A hard, painless lump in the neck, armpit or groin, especially with weight loss, long-lasting fever or night sweats.
These features do not always mean cancer, but they are reasons to have a doctor examine the lump and possibly take a biopsy, to make sure conditions such as a soft tissue tumor or abnormal lymph nodes are ruled out.
Evidence-based treatment of lumps under the skin
Treatment depends on the type of lump, its size, symptoms and the patient's concerns. The approaches dermatologists use include:
Observation: a small cyst or lipoma that has no symptoms, is not growing and has a clear diagnosis can be monitored without surgery.
Complete surgical excision: the permanent treatment for cysts (removing the whole sac wall so it does not return) and for lipomas that are bothersome or growing, done as a minor procedure under local anesthesia.
Incision and drainage: for a cyst that is acutely infected and inflamed. This is temporary relief, not a permanent treatment, and is usually followed by surgical removal of the sac later once the inflammation settles.
Intralesional corticosteroid injection: helps reduce inflammation of a cyst in some cases.
Biopsy: when a lump has features that need monitoring, to establish a definite diagnosis before planning treatment.
Surgery for lumps under the skin should be done by a doctor in a facility that meets proper standards, so the lump is removed completely, the chance of recurrence is reduced and the scar is looked after as well as possible.
Debunking myths about lumps under the skin
"A lump under the skin is cancer": most are benign growths, but some features need to be checked for reassurance (see the warning signs above).
"Squeeze the fat out and the lump will go away": the sac wall remains, the lump always returns, and there is a risk of infection.
"Fatty cysts come from eating fatty food": most epidermoid cysts have nothing to do with diet or blood fats.
"Leave it and it will shrink by itself": cysts and lipomas usually do not go away on their own. They may stay the same or slowly grow. The decision to treat depends on symptoms and diagnosis.
"A painless lump can be ignored": many dangerous lumps, especially hard, fixed, fast-growing ones, are often painless in the early stage.
Frequently asked questions about lumps under the skin
Which lumps under the skin are dangerous?
A lump that is very hard, firmly fixed and immovable, growing fast, larger than 5 centimeters, painful without infection, or with an ulcerated skin surface is a sign to see a doctor promptly. A lump that is soft, movable, slow-growing and painless is usually benign.
How does an epidermoid cyst differ from a fatty lump (lipoma)?
An epidermoid cyst is usually firm, attached to the skin, has a central opening, and may release thick, foul-smelling material when inflamed. A lipoma is soft and rubbery, rolls around, has no opening and releases nothing, because it is solid fatty tissue.
Does a lump under the skin need to be removed?
Not always. If the lump is small, has no symptoms and the diagnosis is clear, it can be monitored. Removal is advisable when the lump grows, interferes with daily life, becomes repeatedly infected, sits where it is pressed on, or the diagnosis is still uncertain.
Can I squeeze a cyst myself?
No. Squeezing removes only the contents temporarily, the sac wall remains and the lump comes back. Squeezing can also rupture the sac, causing inflammation and infection that make later treatment harder.
Can a lump under the skin go away by itself?
Cysts and lipomas usually do not go away on their own. A pus-filled lump from infection may shrink once the inflammation is treated, but if it is a cyst the sac remains and can become inflamed again. The permanent treatment is removing the sac.
Which specialist should I see for a lump under the skin?
A dermatologist can diagnose and remove common lumps under the skin, such as cysts and lipomas, and can take a biopsy. If a lump has features that need monitoring, the doctor will assess it and order further tests or refer you to the appropriate specialist.
Written by Dr. Patara Voravutinon, Dermatologist · Last reviewed August 24, 2026
Evidence-based references: British Association of Dermatologists (BAD) — Cysts, epidermoid and pilar (states clearly that these cysts are not "sebaceous cysts"); DermNet — Epidermoid cyst, Lipoma, Steatocystoma multiplex; Trinh CT et al., Overview of epidermoid cyst (2019). This article provides general information and is not a diagnosis or an individual treatment order. If you are concerned about a lump, see a doctor for an examination.
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