A clinical and histologic study of recurrent pilonidal sinus is presented. During a 5-year period 44 patients were treated for recurrent pilonidal sinus. Of these, recurrence had occurred within 1 year in 26 patients (76%). After 26 reoperations with excision and primary suture further recurrence was seen in 23% (6/26), and in 24% (6/25) after reoperations with open treatment. The histologic findings in recurrent sinuses (18 patients) were almost identical to that of primary disease. Recurrent sinuses were situated in the scar in 80% of the cases and caudal in 50% of the cases. A known wound infection had been present in 27% of the patients. Although keratin plugs were observed in 15/18 (83%) of those with a recurrence, the importance of this finding is uncertain. We conclude that recurrent pilonidal sinuses are chronic inflammatory processes usually located at the site of the surgical wound. The cause is uncertain, although there is an indication that wound infection plays a role. Keratin plugs have also been observed in scar tissue. Surgical treatment of recurrent disease has a higher recurrence rate than after that of primary disease.