An increased incidence of carcinoma of the small bowel and colon has been described in patients with Crohn's disease. Tumours arising in the rectum and anus are reported less often. Between 1940 and 1992, of some 2500 patients with Crohn's disease seen at this hospital, 15 are known to have developed carcinoma of the lower gastrointestinal tract. Malignancy occurred in the colon in two patients, in the upper two thirds of rectum in one, in the lower third of rectum in seven, and in the anus in five. The 12 patients with carcinoma arising in the anus or lower rectum had longstanding severe anorectal Crohn's disease, which included a stricture in four, fistula in four, proctitis in one, abscess in two, and enlarged anal skin tags in one. The development of malignancy in patients with Crohn's disease may apply particularly to those with chronic complicated anorectal disease.