A pilonidal sinus is an inflamed tract (or tracts, as there can be more than one) which leads to a cavity under the skin between the buttocks (see diagram). It is thought that the condition is started by hairs growing inwards into the skin. The tract and cavity often become infected, causing a discharge. If an abscess forms, this may cause a swelling and can burst. A pilonidal sinus can cause chronic problems, sometimes over several years. Pilonidal sinus is relatively common and often affects young adults, especially men.