We investigated the pattern of proliferation of epithelial cells in rectal mucosa taken from normal individuals and patients with ulcerative colitis by incubating mucosa with tritiated thymidine in vitro and processing for autoradiography. We found that patients with ulcerative colitis in remission showed a proliferative pattern similar to that seen in both regenerating and "precancerous' mucosa. Patients with a short history were as likely to show this pattern as those with a long history, and this shows that the abnormal pattern does not signify impending malignant change. We also found that mucosa from patients with ulcerative colitis in remission showed an increased proportion of cells synthesising DNA, a proportion surprisingly close to that seen in an active phase; this suggests that the abnormal pattern seen in remission is the pattern of a regenerating mucosa. We feel that this high rate of mucosal turnover, sustained not just during clinically active disease but throughout remission, leads to the increased incidence of carcinoma and to the development of carcinoma in flat mucosa.