A pilonidal sinus is a pit in the skin, often containing hairs, in the upper part of the cleft between the buttocks. The cause is probably a broken hair fragment growing inward into the skin.
A pilonidal sinus is usually harmless, does not cause symptoms, and does not need treatment.
However, it may become infected, which may result in a painful, swollen, pus-filled abscess (called a pilonidal cyst) that does need treatment. Treatment is usually with antibiotics and surgery to drain the pus.
If there are repeated infections, the sinus may be surgically removed.

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