An epidermal cyst is a slow-growing, benign lump that forms just beneath the skin when surface skin cells get trapped and begin producing a protein called keratin in an enclosed pocket. These cysts are among the most common skin growths, and while they’re almost always harmless, they can become inflamed, infected, or simply bothersome enough to warrant removal.
How Epidermal Cysts Form
Your outer layer of skin constantly sheds old cells. Normally, these cells flake off and are replaced by new ones underneath. An epidermal cyst develops when some of those surface cells move deeper into the skin instead of shedding. Once trapped, the cells form the walls of a small sac and continue doing what skin cells do: producing keratin, the same tough protein that makes up your hair and nails. With nowhere to go, the keratin accumulates inside the sac, and the cyst slowly grows.
This is why epidermal cysts are sometimes called “epidermal inclusion cysts,” since the skin cells are essentially included, or embedded, where they don’t belong. The thick, paste-like material inside the cyst is compressed keratin, which often has a strong, unpleasant smell if the cyst ruptures or is drained.
Common Causes
Skin injury or irritation is the most frequent trigger. A cut, scrape, surgical wound, or even chronic friction can push surface cells beneath the skin where they become trapped. Damage to a hair follicle, whether from shaving, acne, or other irritation, can also block the follicle and start cyst formation.
Some people are genetically predisposed. A rare inherited condition called Gardner syndrome increases the likelihood of developing multiple epidermal cysts, sometimes in unusual locations. But for most people, epidermal cysts are isolated, random events with no underlying condition behind them.
What an Epidermal Cyst Looks Like
A typical epidermal cyst appears as a round, dome-shaped bump under the skin. It ranges from about a quarter inch to over two inches in diameter, though most stay on the smaller end. One distinguishing feature is a small dark dot, called a punctum, visible at the center of the lump. This is the remnant of the blocked pore or follicle opening. Not every cyst has an obvious punctum, but when present, it’s a helpful clue.
The cyst usually feels firm but movable. If you press on it gently, you can often shift it slightly under the skin, which distinguishes it from growths that are fixed to deeper tissue. The overlying skin may appear normal, pink, red, or slightly darker than your natural skin tone. Most epidermal cysts are painless unless they become inflamed or infected, at which point they turn tender, warm, and noticeably swollen.
The most common locations are the face, neck, chest, upper back, and stomach, all areas with dense concentrations of hair follicles. They can appear almost anywhere on the body, though they rarely develop on the palms or soles.
Epidermal Cysts vs. Lipomas
People often confuse epidermal cysts with lipomas, since both are slow-growing lumps under the skin. The key difference is what’s inside. A lipoma is a collection of fat cells, so it feels soft and rubbery. An epidermal cyst is filled with keratin, making it firmer. Lipomas also lack the central punctum and tend to sit slightly deeper beneath the skin. Lipomas are typically found on the shoulders, neck, stomach, chest, and back, overlapping with cyst locations, but they almost never produce the cheesy, foul-smelling discharge that characterizes an epidermal cyst.
Pilar cysts are another lookalike. These form specifically on the scalp and develop from the outer lining of a hair follicle rather than from surface skin cells. They contain a denser, more uniform type of keratin and usually lack the central punctum. The distinction matters mainly for treatment planning, since pilar cysts tend to shell out more cleanly during removal.
When Cysts Become Inflamed or Infected
An epidermal cyst can rupture beneath the skin’s surface, spilling its keratin contents into surrounding tissue. Your immune system treats this leaked material as a foreign substance and mounts an inflammatory response: the area becomes red, swollen, painful, and warm. This is a sterile inflammation, not an infection, but it can look and feel identical to one.
True bacterial infection can also develop, either spontaneously or after a rupture. Distinguishing between sterile inflammation and active infection is genuinely difficult even for physicians. Because the two conditions look so similar, many doctors treat inflamed cysts with antibiotics as a precaution, along with draining the contents to relieve pressure and pain.
Squeezing or attempting to pop a cyst at home significantly increases the risk of both rupture and infection. The cyst wall is fragile, and external pressure can tear it open beneath the skin, triggering inflammation while also introducing bacteria from the skin surface or from under your fingernails. A cyst that was painless for months or years can turn into an angry, swollen mass within days of being squeezed.
How Epidermal Cysts Are Diagnosed
Most epidermal cysts are diagnosed by physical examination alone. The combination of a round, mobile lump with a central punctum in a typical location is distinctive enough that further testing is often unnecessary.
When the diagnosis is uncertain, ultrasound imaging can help. On ultrasound, epidermal cysts typically appear as well-defined, dark (hypoechoic) masses with a feature called posterior sound enhancement, where the area directly behind the cyst appears brighter because the cyst contents transmit sound waves easily. About 80% of epidermal cysts show this pattern. Importantly, they don’t show increased blood flow on Doppler imaging, which helps distinguish them from solid tumors that recruit their own blood supply. If a cyst has ruptured, the ultrasound may show scattered bright spots within the mass and increased blood flow around the cyst wall rather than inside it.
Treatment and Removal
Epidermal cysts that are small, painless, and not bothersome don’t require treatment. Many people live with them indefinitely without issues.
When removal is desired, whether for cosmetic reasons, discomfort, or recurrent inflammation, surgical excision is the standard approach. The critical detail is that the entire cyst wall must be removed. If even a small fragment of the lining remains, the cyst can regenerate from that remnant. This is the most common reason cysts recur after treatment: incomplete removal of the sac.
For inflamed or infected cysts, doctors typically address the inflammation first with drainage and sometimes antibiotics, then schedule the full excision for a later date once the tissue has calmed down. Attempting to remove an actively inflamed cyst is more difficult because the swollen, fragile wall is harder to extract in one piece, raising the chance of recurrence.
A minimal excision technique uses a smaller incision than traditional surgery, resulting in less scarring. A small punch or cut is made over the cyst, the contents are expressed, and then the cyst wall is pulled out through the opening. Recovery from either approach is straightforward, typically involving wound care for one to two weeks and a small scar at the site.
Malignant Transformation Risk
The vast majority of epidermal cysts remain benign for life. Malignant transformation does occur, but it’s rare, and the true frequency is debated. Estimates in the medical literature range from less than 1% to as high as 20%, a wide spread that reflects differences in study populations and how cases are identified. When cancer does develop within a cyst, squamous cell carcinoma is by far the most common type.
Rapid growth, a sudden change in appearance, fixation to deeper tissue (meaning the lump no longer moves freely), or a cyst that keeps recurring after complete removal are all signs that warrant closer evaluation. In practice, any cyst removed surgically is sent for pathology analysis, which catches the rare cases where abnormal cells are present.