The incidence and prognosis of anal fistulas were investigated in a prospective study comprising 136 patients operated on for Crohn's disease. The incidence of anal fistulas was 27 of 136 (20%), in patients with classical disease 12 of 68 (18%), and in those with Crohn's colitis 15 of 68 (22%). A fistula preceded the intestinal manifestation of the disease in 6 patients. At the time of diagnosis of Crohn's disease anal fistulas were observed in 19 cases, including 3 of the 6 with early onset which had resisted treatment and remained active. Five patients developed anal lesions during the course of the disease, all but 2 in temporal relationship to an intestinal recurrence. Of the 27 patients with anal fistulas, 11 were of the low-anal type, whereas 14 were anorectal. Conventional laying open of the fistula was undertaken in all patients with classical Crohn's disease in close conjunction with resection of the intestinal disease. On local surgical treatment 10 of 12 (89%) healed with preservation of continence. Four recurrent fistulas occurring in conjunction with intestinal recurrence also healed uneventfully. Laying open was undertaken in 11 of the 15 patients with colitis. Healing was obtained in only 4 of these patients. In the remaining 4 severe colitis indicated immediate proctocolectomy. Occurrence of fistulas involved a significant delay in perineal healing after proctectomy. It is concluded that traditional laying open of an anal fistula in patients with classical Crohn's disease is followed by high rate of uneventful healing. In contrast, local surgical treatment of anal fistulas complicating Crohn's colitis is usually unsuccessful.(ABSTRACT TRUNCATED AT 250 WORDS)