This study is undertaken to evaluate the efficacy and safety of rioprostil, 300 micrograms, compared with ranitidine, 150 mg, when given twice daily for 4-6 weeks to patients with active, uncomplicated duodenal ulcer. The effects of each drug on ulcer healing are evaluated by endoscopy. Of a total of 355 patients who have entered this study, 319 are statistically evaluated for efficacy; 162 receive rioprostil and 157 receive ranitidine. After 4 weeks of treatment, 63% of the patients receiving rioprostil are endoscopically healed, compared with 72% of those receiving ranitidine. After 6 weeks of treatment, the cumulative healing rates are 86% and 93.5% respectively; this difference is statistically significant. Diarrhoea is the main adverse event, but is generally mild and self-limiting. These results indicate that rioprostil, 300 micrograms b.d., is a safe and effective treatment for duodenal ulcer, but is slightly less effective than ranitidine, 150 mg b.d.