You may be more likely to have a pilonidal cyst if you were born with a little dimple in the skin between the buttocks. The ingrown hair irritates the skin and causes a cyst to form around the hair. This might happen, for example, because of pressure or friction, like when you have been sitting or riding a bicycle for a long time. A hair grows back under the skin or skin grows closed over a hair. These cysts seem to be caused most often by an ingrown hair. It can get infected and cause an abscess, which is a pocket of infected fluid (pus) with a thick wall around it. It often contains hair and small pieces of skin. It usually happens in the crease between the buttocks, near the tailbone. Pilonidal Cyst What is a pilonidal cyst? A pilonidal cyst is an abnormal saclike structure in the skin.
