OBJECTIVE Persistent perineal sinus is a source of morbidity after proctocolectomy for Crohn's disease. This study examined the factors responsible for persistent sinus after proctocolectomy for Crohn's disease. We also assessed the outcome of surgical treatment for persistent perineal sinus. METHODS The records of 145 patients who underwent proctocolectomy for Crohn's disease between 1970 and 1997 were reviewed. RESULTS Persistent sinus occurred in 33 (23 percent) patients after proctocolectomy. Factors associated with a significantly greater risk of perineal sinus were younger age (P = 0.006), rectal involvement (P = 0.02), perianal sepsis (P = 0.0005), high fistulas (P = 0.04), extrasphincteric excision (P = 0.0004), and fecal contamination at operation (P = 0.0003). Multivariate analyses showed that age (P = 0.0001), rectal involvement (P = 0.007), and fecal contamination (P = 0.009) were significant independent predictive factors for perineal sinus. Fifty-six operations, including 24 radical excisions, two rectus abdominis flaps, four gracilis transpositions, and two omentoplasties were performed in 24 patients with persistent sinus, but only 9 achieved healing. Long sinuses (>10 cm) and sinuses presenting late (>12 weeks after proctocolectomy) were seldom cured by surgical treatment. CONCLUSIONS Persistent perineal sinus is more likely to occur if an extrasphincteric dissection is needed because of extensive anorectal disease or if fecal contamination occurs at operation. Attempted surgical eradication of perineal sinus is often ineffective.