In this double-blind parallel group multicenter international study, patients with endoscopically proven acute duodenal ulcers were allocated randomly to treatment for four weeks with either misoprostol 50 micrograms, misoprostol 200 micrograms or cimetidine 300 mg, each given q.i.d.. Endoscopic, clinical and laboratory assessments were made prior to and after four weeks' treatment. A clinical assessment was also made at two weeks. 703 patients were recruited. The three treatment groups were similar with regard to age, sex and occupation. Therapeutic success was defined as complete healing of all ulcers on the basis of an endoscopic examination. On an intent to treat basis, which includes all losses to follow-up and withdrawals as treatment failures, the cure rates in the misoprostol 50 micrograms, misoprostol 200 micrograms and cimetidine groups were 41%, 60% and 67% respectively. There was no statistically significant difference between the misoprostol 200 micrograms and the cimetidine 300 mg groups (P = 0.11). Both were significantly better than the low dose misoprostol group (P less than 0.001). All three treatments were well tolerated and severe adverse events were rare.