The efficacy of misoprostol (a synthetic analogue of prostaglandin E1) and cimetidine in the treatment of duodenal ulcer was evaluated. Seventy-one patients with endoscopically proven duodenal ulcer were randomized in a double-blind manner in one of three groups that received four daily doses of either misoprostol, 50 or 200 micrograms, or cimetidine, 300 mg. Ulcer healing was assessed endoscopically after 4 weeks of treatment. The mean age, sex distribution, and tobacco, alcohol and caffeine consumption were similar in all treatment groups. Only one patient was lost to follow-up. On the misoprostol low dose, healing of the ulcer was observed in 60.9% of the patients. In contrast, healing on the higher dose of misoprostol was not significantly different to that with cimetidine. No significant clinical or laboratory side effects were observed in any of the patients. We found that the daily dose of 800 micrograms misoprostol is safe and effective in the treatment of duodenal ulcer.