The efficacy and safety of famotidine (40 mg at night), a new potent H2-receptor antagonist, has been studied in 119 patients by four investigators in four Spanish hospitals in a randomized double-blind comparative study with cimetidine (800 mg at night). Antacid tablets were allowed as additional treatment, if needed for pain relief. There were no significant differences between the groups in baseline characteristics, including duodenal ulcer size. Efficacy parameters included daytime and nocturnal symptom relief and duodenal ulcer healing, documented by endoscopy, and defined as complete reepithelization of the ulcer crater. Endoscopy was performed at baseline and after 4 and 6 weeks of treatment. One hundred and five patients fulfilled the evaluation criteria (51 patients in the famotidine group and 54 in the cimetidine group). After 4 weeks, in 91.6% of the patients receiving famotidine and 82.3% of the patients receiving cimetidine ulcers were healed. After 6 weeks, healing rates were 96% (famotidine) and 85.1% (cimetidine) (p = 0.056). Pain relief was rapid in both treatment groups, with a tendency to better response during the day in the famotidine group. The intake of antacids, as well as the clinical and laboratory safety profile were similar for both groups.