Forty-five of 47 patients with distal ulcerative colitis completed a two-week double-blind, randomized, controlled trial to determine if 4-aminosalicylic acid (4-ASA) enemas, 1 g bid or 2 g bid, were therapeutically effective compared to placebo. Forty-one patients enrolled because they were refractory to or had side effects during conventional therapy with sulfasalazine or corticosteroids. Proctoscopic examination was done before and after two weeks of treatment. Patients kept daily diaries assessing: blood in stools, mucus in stools, tenesmus, abdominal pain, loss of appetite, fatigue, weight loss, and malaise. Severity of each symptom ranged from 0 (absent) to 3 (severe). A total severity score was calculated from the above for each patient. At the end of the two-week study, 35 patients elected to take 4-ASA in an open-label trial for one year. 4-ASA enemas in the 1-g bid but not the 2-g bid dosage were significantly more effective in improving symptoms than placebo: P less than or equal to 0.05. Neither dose of 4-ASA enema was better than placebo in improving the sigmoidoscopic appearance at the end of two-weeks. Forty-six percent of patients had complete resolution of all signs and symptoms in the open-label trial and 31% were better but still had sigmoidoscopic evidence of disease, a total response rate of 77%. Side effects were similar in the placebo and 4-ASA groups. We conclude that 4-ASA enemas in a dose of 1 g bid are safe and effective in the treatment of distal ulcerative colitis.