A sebaceous cyst is a term commonly used to refer to either: Epidermoid cysts (also termed epidermal cysts, infundibular cyst) Pilar cysts (also termed trichelemmal cysts, isthmus-catagen cysts) Both of the above types of cysts contain keratin, not sebum, and neither originates from sebaceous glands. Epidermoid cysts originate in the epidermis and pilar cysts originate from hair follicles. Technically speaking, then, they are not sebaceous cysts. "True" sebaceous cysts, which originate from sebaceous glands and which contain sebum, are relatively rare and are known as steatocystoma simplex or, if multiple, as steatocystoma multiplex. Medical professionals have suggested that the term "sebaceous cyst" be avoided since it can be misleading. In practice, however, the term is still often used for epidermoid and pilar cysts. Signs and symptoms The scalp, ears, back, face, and upper arm, are common sites of sebaceous cysts, though they may occur anywhere on the body except the palms of the hands and soles of the feet. They are more common in hairier areas, where in cases of long duration they could result in hair loss on the skin surface immediately above the cyst. They are smooth to the touch, vary in size, and are generally round in shape. They are generally mobile masses that can consist of: Fibrous tissues and fluids A fatty (keratinous) substance that resembles cottage cheese, in which case the cyst may be called "keratin cyst" - this material has a characteristic "cheesy" or foot odor smell A somewhat viscous, serosanguineous fluid (containing purulent and bloody material) The nature of the contents of a sebaceous cyst, and of its surrounding capsule, differs depending on whether the cyst has ever been infected. With surgery, a cyst can usually be excised in its entirety. Poor surgical technique, or previous infection leading to scarring and tethering of the cyst to the surrounding tissue, may lead to rupture during excision and removal. A completely removed cyst will not recur, though if the patient has a predisposition to cyst formation, further cysts may develop in the same general area. Causes Cysts may be related to high levels of testosterone, hence may be more frequent in users of anabolic steroids. A case has been reported of a sebaceous cyst being caused by the human botfly. Hereditary causes of sebaceous cysts include Gardner's syndrome and basal cell nevus syndrome. Types Epidermoid cyst Pilar cyst About 90% of pilar cysts occur on