Pilar cysts are firm, round bumps that grow just beneath the skin, most commonly on the scalp. They form from the outer root sheath of a hair follicle and are filled with keratin, the same tough protein that makes up your hair and nails. They’re benign, usually painless, and grow slowly over months or years. About 90% of pilar cysts show up on the scalp, though they can appear anywhere you have hair.
What Causes Pilar Cysts
A pilar cyst develops when cells from the hair follicle’s outer root sheath create a sac that fills with keratin and its breakdown products. The sac sits within the skin itself, nestled between the oil gland and the tiny muscle that makes your hair stand up. Unlike a pimple or abscess, the cyst has a distinct wall (called a capsule) that separates it from the surrounding tissue. This capsule is what gives pilar cysts their characteristic firm, smooth feel when you press on them.
In many cases, pilar cysts run in families. They follow an autosomal dominant inheritance pattern, meaning if one of your parents has them, you have roughly a 50% chance of developing them too. If you have one pilar cyst, you’re likely to develop more over time. Multiple cysts on the scalp are common in people with the inherited form.
Who Gets Them
Pilar cysts are the second most common type of skin cyst, accounting for about 15% of all cutaneous cysts in large studies. Women develop them more often than men, at a ratio of roughly 60 to 40. They most frequently appear during the fourth decade of life (ages 31 to 40), though they can show up at any age. In a study of over 1,100 skin cysts, the scalp was the location in about 74% of pilar cyst cases.
How They Feel and Look
Most pilar cysts are smooth, dome-shaped lumps that move slightly under the skin when you push on them. They typically range from pea-sized to a few centimeters across, though some grow larger over years. The overlying skin usually looks normal, with no redness or discoloration. Because they’re so common on the scalp, many people discover them while washing or brushing their hair.
Pilar cysts are usually painless. They can become tender if they get inflamed or infected, which sometimes happens if one ruptures beneath the skin or is irritated by repeated pressure (like from a hat or helmet). An infected cyst may become red, swollen, and warm to the touch, and it can drain a thick, foul-smelling material. This is the keratin contents leaking out, not pus from a bacterial infection, though secondary infection can develop.
Pilar Cysts vs. Epidermoid Cysts
People often confuse pilar cysts with epidermoid cysts (sometimes incorrectly called sebaceous cysts). Both are keratin-filled lumps under the skin, but they originate from different parts of the hair follicle and look different under a microscope. Epidermoid cysts come from the upper portion of the follicle and have a lining that includes a granular layer, producing keratin in thin sheets. Pilar cysts lack that granular layer entirely and produce dense, compact keratin instead.
The practical differences: epidermoid cysts can appear almost anywhere on the body, while pilar cysts strongly favor the scalp. Pilar cysts also tend to have a thicker, tougher capsule, which is actually good news if you need one removed, because it often pops out cleanly in one piece. Epidermoid cysts have thinner walls that are more likely to tear during removal.
When Pilar Cysts Need Attention
Most pilar cysts don’t require treatment. Many people live with them for years without any problems. Removal is typically considered when a cyst is painful, repeatedly gets inflamed, grows large enough to be bothersome, or is cosmetically concerning.
In rare cases, a pilar cyst can develop into what’s called a proliferating trichilemmal tumor. This is a more aggressive growth where the cells lining the cyst begin multiplying rapidly. These tumors tend to appear on the scalp in women between their 40s and 80s. They grow slowly but can become locally invasive and occasionally ulcerate. While proliferating trichilemmal tumors are generally considered benign, there are documented cases of them transforming into squamous cell carcinoma, a type of skin cancer. Any pilar cyst that suddenly starts growing quickly, becomes fixed to the underlying tissue, or develops an ulcerated surface should be evaluated promptly.
How Removal Works
The standard treatment for a pilar cyst is complete surgical excision, typically done as an outpatient procedure under local anesthesia. Your doctor numbs the area, then makes a small incision directly over the cyst. Because pilar cysts have that thick, fibrous capsule, the surgeon can often remove the entire cyst intact through blunt dissection, essentially teasing it away from the surrounding tissue without cutting into it. If the cyst is large, the contents may be expressed first and the capsule wall removed afterward.
Getting the entire capsule out is critical. If any portion of the cyst wall is left behind, the cyst will likely grow back in the same spot. This is the most common reason for recurrence after removal. Proliferating trichilemmal tumors are also cured with complete excision in most cases.
Recovery After Removal
Scalp cyst removal generally heals well, since the scalp has excellent blood supply. You’ll typically have stitches that come out at a follow-up visit 7 to 10 days after the procedure. Small cysts that don’t require stitches heal within a few days to two weeks. Larger cysts with bigger incisions can take several weeks to fully heal. During recovery, keeping the wound clean and dry is the main priority, and your doctor will give you specific instructions about when you can resume washing your hair and what to watch for in terms of infection.
Scarring is usually minimal on the scalp because hair covers the site. For cysts in visible locations, the size of the scar depends on how large the cyst was and the technique used. Smaller cysts removed through punch incisions leave very little trace.