Three patients with Crohn's disease and carcinoma of the anus are reported and compared to a group of patients with anal cancer and no inflammatory bowel disease. The three patients with Crohn's disease were relatively young women with significant perianal disease. There were two squamous cell lesions and one cloacogenic tumor in this group. The relative incidence of anal cancer as a proportion of all colorectal cancer, in patients with Crohn's disease (14%) was found to be significantly higher than the incidence of anal cancer in patients without inflammatory bowel disease (1.4%). Possible reasons for the increased incidence of anal cancer in Crohn's disease mentioned were: an overall increase in malignancies in inflammatory bowel disease, the high incidence of perianal disease, and the chronic long-standing perianal inflammation present. All patients with Crohn's disease, especially if they have perianal disease, should be observed for the occurrence of anal cancer.