A Pilonidal Sinus is a small hole or tunnel in the skin at the top of the buttocks near the tailbone, where they divide (the cleft). The sinus tract goes in a vertical direction between your buttocks. A pilonidal cyst usually contains hair, dirt, and debris. It can cause severe pain and can often become infected and filled with pus. A Pilonidal Sinus can go unnoticed unless it becomes infected. Pilonidal sinuses are more common in men because they tend to be hairier. Commonly found in teenagers and young adults.