A pilonidal sinus is a small tunnel that develops at the top of the crease in the buttocks. Trapped hair around the buttocks crease can lead to an abscess, and a pilonidal sinus may develop as a ...

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.

From the Department of Surgery, Peter Bent Brigham Hospital, Boston, Massachusetts. Glenn — On staff of the New York Hospital, New York City. For record and address of author see "This Week's Issue," ...