Three hundred and twenty two patients with pilonidal sinus disease were studied to determine factors for the development and maintenance of the disease. A calculated incidence of the disease of 26 per 100,000 inhabitants was found. It occurred 2.2 times more often in men than in women. Age at presentation was 21 years for men and 19 for women. Patients had two years (median) disease history before being referred for treatment. A family history could be found in 38% of the patients. 50% had normal body weight, and 37% were overweight. Local trauma or irritation preceded the condition in 34%, and a sedentary occupation was reported by 44%. Male sex, adolescence or youth, and a familial disposition seem to be associated with the development of pilonidal sinus. Local trauma and overweight are the most important conditioning factors for development of symptomatic pilonidal sinus disease.