Epidermoid cyst signs and symptoms include:

• A small, round bump under the skin, usually on the face, neck or trunk
• A tiny blackhead plugging the central opening of the cyst
• A thick, yellow, smelly material that sometimes drains from the cyst
• Redness, swelling and tenderness in the area, if inflamed or infected

The surface of your skin (epidermis) is made up of a thin, protective layer of cells that your body continuously sheds. Most epidermoid cysts form when these cells move deeper into your skin and multiply rather than slough off. Sometimes the cysts form due to irritation or injury of the skin or the most superficial portion of a hair follicle.

The epidermal cells form the walls of the cyst and then secrete the protein keratin into the interior. The keratin is the thick, yellow substance that sometimes drains from the cyst. This abnormal growth of cells may be due to a damaged hair follicle or oil gland in your skin.

Many people refer to epidermoid cysts as sebaceous cysts, but they're different. True sebaceous cysts are less common. They arise from the glands that secrete oily matter that lubricates hair and skin (sebaceous glands).

Nearly anyone can develop one or more epidermoid cysts, but these factors make you more susceptible:

• Being past puberty
• Having certain rare genetic disorders
• Injuring the skin

Potential complications of epidermoid cysts include:

• Inflammation. An epidermoid cyst can become tender and swollen, even if it's not infected. An inflamed cyst is difficult to remove. Your doctor is likely to postpone removing it until the inflammation subsides.
• Rupture. A ruptured cyst often leads to a boil-like infection that requires prompt treatment.
• Infection. Cysts can become infected and painful (abscessed).
• Skin cancer. In very rare cases, epidermoid cysts can lead to skin cancer.