An epidermoid cyst is a slow-growing, noncancerous lump that forms just beneath the skin when skin cells get trapped below the surface and continue producing keratin, the same tough protein that makes up the outer layer of your skin. Instead of shedding normally, these cells accumulate inside a sac, gradually filling it with soft, paste-like material. Most epidermoid cysts are small, painless, and harmless, but they can become inflamed or infected if they rupture beneath the skin.
How Epidermoid Cysts Form
The vast majority of epidermoid cysts originate from the upper portion of a hair follicle. When that follicle is damaged, irritated, or blocked, skin cells from the follicle lining can become enclosed beneath the surface. These trapped cells behave just as they would on the skin’s outer layer: they multiply and produce keratin. But with nowhere for the keratin to go, it builds up in layers inside a sac, and the cyst slowly grows over weeks to months.
A smaller number of epidermoid cysts form through other mechanisms. Some develop when skin cells become embedded during an injury or surgery, essentially being pushed into deeper tissue where they continue growing. Others form from tiny clusters of skin cells that were misplaced during embryonic development. In rare cases, certain strains of HPV have been linked to cyst formation.
What They Look and Feel Like
A typical epidermoid cyst feels like a small, firm, round bump that you can move slightly under the skin. Most range from pea-sized to a few centimeters across, though some grow larger over time. They show up most often on the face, neck, and trunk, but they can appear nearly anywhere on the body.
One of the most distinctive features is a tiny dark dot at the center of the cyst, sometimes described as a small blackhead plugging the opening. This is the remnant of the blocked pore or follicle from which the cyst developed. If you squeeze the cyst (which is not recommended), it may release a thick, yellowish, foul-smelling material. That material is the accumulated keratin.
Epidermoid Cysts vs. “Sebaceous Cysts”
Epidermoid cysts are commonly called sebaceous cysts, but that name is misleading. True sebaceous cysts arise from oil-producing glands in the skin and are actually quite rare. Epidermoid cysts come from ordinary skin cells, not sebaceous gland cells. The British Association of Dermatologists notes that the term “sebaceous cyst” has been widely dropped by specialists for this reason.
Another lookalike is the pilar cyst, which tends to appear on the scalp. The difference is in the lining of the sac: epidermoid cysts are lined with cells identical to the outer skin surface, while pilar cysts are lined with cells resembling those found in hair roots. From the outside, both feel similar, so the distinction is usually made by examining the tissue after removal.
When Cysts Become a Problem
Most epidermoid cysts cause no symptoms beyond a visible bump. Problems arise when the cyst wall ruptures beneath the skin. When keratin leaks into surrounding tissue, your immune system treats it as a foreign substance, triggering an inflammatory reaction. The area becomes red, swollen, warm, and tender, sometimes dramatically so. This can look identical to an infection, though in many cases it is a sterile inflammatory response rather than a bacterial one. Actual bacterial infection can also develop, particularly if the cyst has been squeezed or picked at.
Malignant transformation, where a cyst develops into skin cancer, has been documented but is exceptionally rare. The clinical features of this transformation are not well understood precisely because it happens so infrequently. A cyst that suddenly begins growing rapidly, becomes fixed to deeper tissue, or develops an ulcerated surface warrants evaluation.
How Epidermoid Cysts Are Diagnosed
Most epidermoid cysts are diagnosed on physical examination alone. The combination of a moveable lump beneath the skin, a central pore, and a characteristic location is usually enough. When there is any uncertainty, ultrasound can help distinguish an epidermoid cyst from other soft tissue masses. On ultrasound, epidermoid cysts typically sit within the skin’s deeper layer (the dermis) and show a characteristic feature where a portion of the cyst projects upward toward the skin surface, connecting to the epidermis. This pattern, along with the cyst’s oval shape and internal texture, helps confirm the diagnosis without a biopsy.
Removal Options
Epidermoid cysts that are not causing symptoms do not require treatment. Many people choose removal for cosmetic reasons or because the cyst is in an area where it gets irritated by clothing or movement.
The standard approach is surgical excision, and there are two main techniques. Conventional excision involves cutting an elliptical (oval-shaped) incision around the cyst and removing the entire sac intact. This takes roughly 11 minutes on average and leaves a slightly larger scar, but it gives the surgeon a clear view of the cyst wall. Minimal excision uses a much smaller opening, typically 3 to 5 millimeters. The surgeon makes a small cut, expresses the cyst contents through the opening, and then pulls the collapsed sac out. This takes about 6 minutes and produces a shorter wound.
Both techniques have similarly low recurrence rates, around 3 percent. The key to preventing recurrence with either method is removing the entire cyst wall. When the wall is completely removed, recurrence rates stay in the single digits. When part of the wall is left behind, recurrence is dramatically more likely. A study of surgically treated cysts in a different anatomical context found a 9 percent recurrence rate when the cyst was considered totally removed, compared to 93 percent when removal was incomplete. The same principle applies to skin epidermoid cysts: any lining left behind can regenerate the cyst.
What to Expect After Removal
Recovery after excision is straightforward for most people. The area is closed with stitches, which are typically removed within one to two weeks depending on the location. Scarring is minimal, especially with the smaller-incision technique, and fades over several months. Most people return to normal activities within a day or two, though cysts in high-movement areas like the back or groin may take a bit longer to heal comfortably.
If a cyst is actively inflamed at the time of your visit, your doctor may recommend treating the inflammation first and scheduling removal for a later date. Removing an inflamed cyst is more difficult because the tissue is swollen and fragile, making it harder to extract the wall intact and increasing the chance of recurrence.
Leaving a Cyst Alone
Epidermoid cysts are benign, and many people live with them for years without any issues. They may slowly grow over time, stay the same size, or occasionally shrink. The main risks of leaving one in place are the possibility of rupture and inflammation, which can happen spontaneously or from pressure on the area. A cyst that has ruptured once is more likely to become problematic again. If you have a cyst that has been stable and painless for a long time, there is no medical urgency to remove it.