In a follow-up study of an epidemiologically defined patient group comprising 1,274 patients with ulcerative colitis diagnosed in Stockholm County during 1955-1979, 55 patients had undergone colectomy with ileorectal anastomosis (IRA). Nine of these were found to have Crohn's disease after histopathologic review of the colectomy specimens. Of the 46 patients with ulcerative colitis remaining for evaluation, two died postoperatively. Twenty-five patients were subsequently reoperated with rectal excision owing to intractable inflammatory activity (n = 22, one postoperative death) or owing to dysplasia (n = 3). Of 19 patients with their IRA still intact at time of follow-up, 15 patients (median disease duration 23 years) had a flexible sigmoidoscopy with multiple biopsies performed. The average length of the remaining rectum and sigmoid colon was 26 cm. No patient had findings of dysplasia, carcinoma, or DNA aneuploidy. None of the four remaining patients had developed dysplasia or carcinoma at the time of the latest regular rigid sigmoidoscopy. The risk of malignant transformation in this selected group of patients with ulcerative colitis operated upon with colectomy and IRA derived from an epidemiologically defined population seems to be low.