A double-blind 4-week trial of cimetidine (1 g daily) versus placebo was conducted in 68 adult outpatients with active duodenal ulceration endoscopically confirmed. 3 of the patient admitted to the study (1 on cimetidine and 2 on placebo) did not complete the trial. After 4 weeks of treatment, 25 of 33 patients (76%) receiving cimetidine had healed their ulcers compared with 15 (47%) of 32 patients receiving placebo (p less than 0.02). Antacid consumption was also significantly decreased by cimetidine when compared with placebo. Tolerability of the drug was good.