Thirty patients with distal ulcerative colitis in remission (17 proctitis, 13 proctosigmoiditis) were randomly given either 5-aminosalicylic acid (5-ASA) or placebo suppositories, 400 mg bid. During the 1-yr follow-up, patients were assessed clinically every month, and flexible sigmoidoscopy with a rectal pinch biopsy specimen and laboratory data were carried out every 3 months. Two patients in the 5-ASA group chose to withdraw from the study, one relapsed, and 12 remained in remission. In the placebo group, one patient chose to withdraw, 11 relapsed, and three remained in remission. The cumulative remission rate at the 12th month was 92% in the 5-ASA group and 21% in the placebo group. Log rank test showed a significant difference in the relapse rate between the two groups (chi 2 = 14.26, p less than 0.001). No side effects were observed. We conclude that 5-ASA in suppository form (800 mg/day), administered for 1 yr, is safe and effective in maintaining remission of distal ulcerative colitis.