Twenty-five patients with active left-sided ulcerative colitis were randomly assigned to receive either 2 g of 4-aminosalicylic acid (para-aminosalicylic acid) or placebo in a 60-mL volume as a nightly retention enema. The duration of treatment was 8 weeks. Disease activity was assessed by grading clinical symptoms of blood, mucus, urgency, sigmoidoscopic findings, and degree of histologic inflammation in rectal biopsies. At 8 weeks, 10 of 12 patients (83%; 95% confidence interval [CI], 55% to 97%) who received 4-aminosalicylic acid showed improvement in clinical, sigmoidoscopic, and histologic variables. In contrast, only 2 of 13 patients (15%, 95% CI, 4% to 38%) who had received placebo showed clinical improvement (P less than 0.005). The 11 patients in the placebo group who showed no improvement were treated subsequently with open-label 4-aminosalicylic acid enemas. Of the 11, 9 showed clinical, sigmoidoscopic, and histologic improvement. No adverse effects were seen. 4-Aminosalicylic acid enemas are a safe and effective means of treating left-sided ulcerative colitis.