There is little information on the natural history of anal fissure in Crohn's disease. The case notes of all new patients with Crohn's disease attending one hospital between 1977 and 1983 were reviewed: there were 61 patients with this diagnosis and an unhealed anal fissure as the only anal lesion. Of these 61 patients the fissure healed in 42 (69 per cent) during medical treatment of the intestinal disease. Ten patients (16 per cent) developed other anal lesions and in the remaining nine patients the fissure remained unhealed at the time of rectal excision (six) or last out-patient attendance (three). Activity of the fissure did not reflect disease activity elsewhere in 11 cases in whom healing of the fissure occurred despite progressive intestinal disease subsequently requiring resection. This study validates the policy of conservative management of anal fissure in Crohn's disease with anal surgery (required in 9.8 per cent of patients in this series) being reserved for the development of other anal disease.