The pharmacotherapy of bleeding peptic ulcer is directed at attempting to keep the gastric pH above the proteolytic range for pepsin. In this randomized, prospective, open clinical trial we have compared the effects and outcome of omeprazole versus ranitidine in patients with bleeding peptic ulcer. Of 219 consecutive patients with upper gastrointestinal bleeding, 51 (23.2%) had an ulcer with endoscopic predictors of rebleeding at the time of diagnosis. These 51 patients were selected at random to receive either omeprazole (80 mg bolus and 40 mg/12 h i.v.) or ranitidine (50 mg/4 h i.v.). No endoscopic therapy was performed at the time of diagnosis. Twenty of these patients with duodenal ulcer (n = 10 omeprazole, n = 10 ranitidine) underwent 24-h gastric pH monitoring. Both groups were homogeneous in all clinical and endoscopic parameters. No differences in blood transfusion units, time of hospitalization, the lowest hematocrit measured, and mortality rates were observed between the groups. However, omeprazole reduced the number of rebleeding episodes (p = 0.1) and the need for surgery (3.8% vs. 22.7%; p = 0.05). Omeprazole also reduced the amount of time the gastric pH was < 6 (15.3 +/- 5.9% vs. 61.8 +/- 5.6%, p < 0.0001). We conclude that parenteral omeprazole is much more effective than ranitidine in keeping the gastric pH above the proteolytic range for pepsin in bleeders and that this might explain a better outcome in a subset of patients with bleeding peptic ulcers treated with parenteral omeprazole.