Epidermoid Cyst

3 min read·Last updated: 2026-02-08·Reviewed by Dr. Julia Tzu, MD, FAAD, FACMS

At a Glance

An epidermoid cyst is a benign, slow-growing bump that forms when keratin-producing cells from the upper layer of skin (epidermis) become trapped beneath the skin's surface. This creates a sac that fills with keratin, a protein that makes up hair, nails, and the outer skin layer. The cyst appears as a round, firm, skin-colored or yellowish lump that can range from a few millimeters to several centimeters in size. While often called "sebaceous cysts," epidermoid cysts contain keratin rather than sebum. They are harmless but can be removed if they become infected, painful, or cosmetically bothersome.

Epidermoid cysts form when keratin-producing epidermal cells become trapped beneath the skin[1]
Complete removal of the cyst including its capsule prevents recurrence[2]
Without treatment, some cysts may remain small and asymptomatic for years[1]
Never attempt to drain or pop a cyst yourself as this risks infection[1]

Symptoms

Round, dome-shaped bump under the skin
Firm but movable lump
Skin-colored, whitish, or yellowish color
Central dark opening (punctum) may be visible
Usually 1-5 centimeters in diameter
Generally painless unless infected
Thick, whitish or yellowish discharge if ruptured
Redness, warmth, and tenderness when infected

When to Seek Professional Care

Signs of infection (redness, pain, swelling, warmth)
Cyst ruptures or drains foul-smelling material
Rapid growth or change in appearance
Location that causes discomfort or interferes with function
Cosmetic concerns
Uncertainty about the nature of the growth

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Causes

Implantation of epidermal cells into dermis
Trauma or injury to skin
Blocked or damaged hair follicle
Developmental abnormality
Acne or skin inflammation
Surgery scars (implantation cysts)
Genetic predisposition

How We Can Help

We offer a service to help manage this condition: Cyst Removal.

Prognosis

Excellent prognosis as these are benign growths
Complete excision with intact capsule prevents recurrence
Incomplete removal may lead to recurrence
Some cysts remain small and stable indefinitely
Rare malignant transformation (extremely uncommon)
Infection is treatable with drainage and antibiotics

Frequently Asked Questions

What is an epidermoid cyst?

An epidermoid cyst is a benign, slow-growing bump that forms when skin cells become trapped under the skin's surface. The trapped cells create a sac filled with keratin, a protein found in skin, hair, and nails. They appear as round, firm lumps that are usually painless unless they become infected.

What is the difference between epidermoid and sebaceous cysts?

The terms are often used interchangeably, but technically differ. Epidermoid cysts contain keratin (a protein) while true sebaceous cysts contain sebum (oil). Most cysts commonly called "sebaceous cysts" are actually epidermoid cysts. Both are benign and treated similarly.

When should I see a doctor about a cyst?

See a doctor if your cyst shows signs of infection (redness, pain, swelling, warmth), ruptures or drains, grows rapidly, or causes discomfort. Also consult a provider if you're unsure what the lump is or if it bothers you cosmetically. Infected cysts need prompt attention.

How are epidermoid cysts removed?

The most effective method is complete surgical excision, removing the entire cyst including its capsule (wall). This is usually done under local anesthesia as an outpatient procedure. Removing the intact capsule prevents the cyst from returning. Simple drainage provides only temporary relief.

What happens if a cyst gets infected?

An infected cyst becomes red, swollen, painful, and warm to touch. It may drain pus or foul-smelling material. Treatment typically involves drainage and antibiotics. After the infection clears, complete excision can be performed to prevent recurrence and future infections.

Can epidermoid cysts be prevented?

Since epidermoid cysts often develop from trapped skin cells or damaged follicles, prevention isn't always possible. Avoiding skin trauma and treating acne may help reduce risk. Once you have one cyst, you may be prone to developing others in different locations.

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