An epidermal cyst, or sebaceous cyst, is a sac beneath the outer layer of the skin that is filled with a cheeselike, greasy material (sebum). These cysts most often appear on the scalp, ears, face, back, or scrotum and are caused by plugged ducts at the site of a hair follicle.

Signs and symptoms include a bump or lump under the skin that is:

• Firm and easily moveable.
• Yellow, white, or flesh-colored. It can turn bright red if injured, inflamed, or infected.
• Painless (but can be painful if injured or infected).
• Between 1 cm (0.4 in.) to 4 cm (1.6 in.) in diameter, although hormone stimulation or injury may cause a cyst to become larger.

Medical treatment is usually not needed if the cyst does not cause symptoms. A cyst can be removed if its appearance causes embarrassment or if it becomes injured or infected. If the cyst is infected, antibiotics may be used.