In this two-center, pilot trial we assessed the efficacy of single bedtime does of 150 mg ranitidine (half dose) and 300 mg ranitidine (full dose) in promoting duodenal ulcer healing by comparing the proportions of healed ulcers after 4 and 8 weeks of treatment. One hundred thirty-nine patients (106 men) were randomly allocated according to a prearranged treatment schedule to either dose and were treated single-blind (endoscopist). One hundred twenty-six patients (63 given 150 mg ranitidine and 63 given 300 mg) completed the trial. In the per-protocol analysis, 55 patients given full doses (81%) and 47 given half doses (70%) of ranitidine had healed ulcers at 4 weeks. Sixty full-dose and 55 half-dose patients (95% and 87%, respectively), had healed ulcers at 8 weeks. The difference was not significant using the chi 2 test (two-tailed), but the 95% confidence limits were in favor of the 300-mg dose. This study had a 75% power to detect a 25% difference in healing rates between the two groups.