A pilonidal cyst is a round sac of tissue that's filled with air or fluid. This common type of cyst is located in the crease of the buttocks and is usually caused by a skin infection. Pilonidal cysts can cause pain and need to be treated. Pilonidal cysts can be a one-time (acute) problem or you may have chronic (returning) cysts. If they’re not treated, chronic pilonidal cysts can also lead to abscesses (swollen pockets of infection) and sinus cavities (empty spaces underneath the skin). A pilonidal cyst can be extremely painful especially when sitting.