A sebaceous cyst is a small, closed sac under the skin that contains oily or cheesy material. The cyst forms a smooth, firm bump or lump, most often on the face, scalp, neck, or middle of the body. A sebaceous cyst may occur if you have a blocked sweat gland or hair follicle. A sebaceous cyst that forms in the top layer of skin cells is called an epidermal cyst. If the cyst sac forms from hair follicle cells, it is called a pilar cyst. Having acne increases your risk for developing a sebaceous cyst. Follow the links below to find WebMD's comprehensive coverage about how sebaceous cyst develops, what it looks like, how to treat it, and much more.