A pilonidal cyst, sometimes called pilonidal disease, is a fluid-filled sac that usually occurs on the tailbone at the top of the buttocks. It usually starts as an infection of a hair follicle in the ...

Pilonidal sinus—also called a pilonidal cyst—is a skin infection that occurs in or around the crease of your buttocks. This condition happens when hair, skin cells, and dirt clog a hair follicle, ...

Also known as a pilonidal cyst, the condition forms when a sac of tissue fills with air or fluid in the crease of your buttocks. Pilonidal cyst, doctors say, is mostly caused by a skin infection.