Colectomy specimens from 62 patients (22 with ulcerative colitis, 20 with Crohn's disease of the colon, and 20 with invasive adenocarcinoma [without inflammatory bowel disease]) were reviewed for the presence of ectopic colonic mucosa. One or more foci of ectopic colonic mucosa were found in 16 of the 22 specimens (72 per cent) with ulcerative colitis and in 11 of the 20 specimens (55 per cent) with Crohn's disease of the colon. None of the 20 specimens having adenocarcinoma (without chronic inflammatory bowel disease) had ectopic colonic epithelium. The presence of ectopic colonic mucosa was found to be dependent on the age of the patients (more frequent among younger patients) and on the number of sections per specimen. One adenocarcinoma in a case of long-standing ulcerative colitis had apparently originated in ectopic colonic mucosa.