A pilar cyst is a firm, round lump that forms beneath the skin, almost always on the scalp. It develops from the outer root sheath of a hair follicle and fills with keratin, the same protein that makes up your hair and nails. Pilar cysts are benign, fairly common (affecting 5 to 10 percent of the population), and usually painless, though they can become a nuisance if they grow large or get irritated.
Where They Form and Why
About 90 percent of pilar cysts appear on the scalp. The remaining cases show up on the face, neck, arms, legs, or scrotum. They originate in the outer layer of the hair follicle, where cells produce keratin in a dense, compact form. When that keratin gets trapped instead of shedding normally, it accumulates inside a self-contained sac just beneath the skin’s surface.
Pilar cysts tend to run in families. The pattern of inheritance is autosomal dominant, meaning if one of your parents had them, you have roughly a 50 percent chance of developing them too. They’re more common in middle-aged adults and occur more frequently in women than in men, with no known difference across racial or ethnic groups.
What a Pilar Cyst Looks and Feels Like
A typical pilar cyst feels like a smooth, flesh-colored marble sitting just under the scalp. Most range from about half a centimeter to five centimeters across, though some grow much larger, occasionally reaching the size of a baseball. The skin over the cyst usually looks normal, with no redness or discoloration unless the cyst has become inflamed.
You can usually move a pilar cyst slightly under your fingers because it sits in the tissue beneath the skin rather than being anchored to deeper structures. Many people discover one while washing their hair. It’s common to have more than one at a time.
How They Differ From Other Cysts
Pilar cysts are often confused with epidermoid cysts (sometimes mistakenly called sebaceous cysts). Both are keratin-filled lumps under the skin, but they differ in a few important ways. Pilar cysts almost always appear on the scalp, while epidermoid cysts are more evenly distributed across the body. Pilar cysts also tend to be odorless, whereas epidermoid cysts often produce a foul smell when they rupture because of their higher fat content.
Under a microscope, the distinction is clear. The wall of an epidermoid cyst contains a visible granular layer filled with tiny protein granules, similar to normal skin. A pilar cyst wall lacks that granular layer entirely, and its keratin is dense and layered rather than flaky. This difference is what a pathologist looks for when confirming a diagnosis after removal.
Diagnosis
Most pilar cysts are diagnosed based on their location, feel, and appearance alone. A doctor can usually identify one during a physical exam by feeling the characteristic smooth, mobile lump on the scalp. When there’s any uncertainty, an ultrasound can help. Pilar cysts appear as well-defined, round structures in the deeper layers of skin, sometimes showing small bright spots from calcium deposits or hair fragments trapped inside.
If a cyst is removed, it’s typically sent to a lab for examination to confirm that it’s benign. This is especially true for cysts that have grown rapidly or look unusual, since a rare variant called a proliferating trichilemmal cyst can, in very uncommon cases, behave more aggressively.
When They Cause Problems
Most pilar cysts sit quietly for years without causing any trouble. Problems tend to arise in a few situations. A cyst can become tender or inflamed if it’s bumped repeatedly, such as during hair brushing. If the cyst wall ruptures beneath the skin, the keratin spills into surrounding tissue and triggers an inflammatory reaction: the area turns red, swollen, and sore. This looks a lot like an infection but is often a sterile inflammatory response rather than a bacterial one.
True infection is possible, especially if you try to squeeze or pop the cyst yourself. An infected pilar cyst becomes warm, increasingly painful, and may drain thick or discolored fluid. Large cysts on the scalp can also be cosmetically bothersome or get caught on combs and hair accessories.
Removal Options
The only permanent treatment for a pilar cyst is surgical removal of the entire cyst, including its wall. Simply draining the contents (incision and drainage) provides temporary relief but frequently leads to recurrence because the sac remains intact and refills with keratin over time.
The most common approach is a minimal excision technique. A small incision of just two to three millimeters is made over the cyst. The contents are expressed through the opening, and then the collapsed cyst wall is carefully grasped and pulled out. The key step is removing every fragment of the sac. If any portion of the wall is left behind, the cyst can grow back. For larger cysts, a traditional full excision with a wider incision and stitches may be necessary to ensure clean removal.
Both procedures are typically done in an office setting under local anesthesia. You’re awake the entire time, and the area is numbed so you won’t feel pain during the procedure itself.
Recovery After Removal
The scalp heals relatively quickly because of its rich blood supply. Most people recover within one to three weeks after a pilar cyst removal. If stitches are placed, they’re generally removed around days four through seven, assuming healing is on track.
During the first few days, you can expect some mild soreness and swelling at the site. Keeping the area clean and dry is the main priority. Hair washing is usually fine within a day or two, depending on the size of the incision. Scarring is typically minimal, especially with the smaller incision technique, and hair growth around the site usually returns to normal once the wound has fully healed.
Cysts That Come Back
Recurrence after complete removal is uncommon. When a cyst does return in the same spot, it almost always means a small piece of the cyst wall was left behind during the original procedure. People who are genetically prone to pilar cysts may also develop new ones in different locations on the scalp over time. Having one removed doesn’t prevent others from forming elsewhere.