A pilonidal cyst is a closed sac under the skin at the top of the crease of the buttocks. Pilonidal cysts usually do not cause problems unless they become infected.

The cause of a pilonidal cyst is unclear. They usually occur in men in their late teens through 20s, although they are occasionally noticed at birth as a depression or sometimes as a hairy dimple.

Symptoms of an infected pilonidal cyst (abscess) include:

• Pain.
• Swelling and redness.
• Drainage of pus and/or blood.
• Fever.

If a pilonidal cyst develops a long-lasting or frequently recurring infection, it is called pilonidal disease. Treatment includes surgical drainage or surgical removal.