Background: Haemostasis is highly pH-dependent and severely impaired at low pH. However, there is no clear evidence that acid-suppressing drugs have beneficial effects in peptic ulcer haemorrhage. Endoscopic haemostatic treatment provides important reduction in morbidity and may be more efficient when a neutral intragastric pH is maintained. Methods: We conducted a double-blind, placebo-controlled multicentre study of intravenous infusion of omeprazole (80 mg as bolus, followed by 8 mg/h) or placebo for 72 h. All patients received 20 mg omeprazole orally from day 3 until follow-up on day 21. Only patients with ulcer haemorrhage, endoscoped within 12 h after admission, and with a history or signs of circulatory failure and spurting bleeding, oozing bleeding, visible vessel, or clot, were included. Endoscopic intervention was aimed at when spurting bleeding, oozing bleeding, or a visible vessel was observed. The primary efficacy measure was the worst ranking on an overall outcome scale (5 = death, 4 = surgery, 3 = additional endoscopic treatment, 2 = more than 3 units of blood, and 1 = no more than 3 units of blood transfused). Base-line prognostic factors of treatment success by day 3 and of other binary outcomes were considered in a logistic regression model. Results: Two hundred and seventy-four patients were randomly assigned to omeprazole (134 patients) or placebo (140 patients). The number of patients included in the 'intention-to-treat' analysis was 130 in the omeprazole group and 135 in the placebo group. The primary variable, the overall outcome at 72 h, showed a difference (P = 0.004) between the two treatments in favour of omeprazole. Treatment success by 72 h defined as no death, no operation, or no additional endoscopic treatment was 91.0% in the omeprazole group and 79.7% in the placebo group (therapeutic gain, 11.3 percentage units; 95% confidence interval, 2.3 to 20.4 percentage units). Significant differences in favour of omeprazole were also found for secondary variables such as number of blood transfusions, duration and degree of bleeding, and the need for surgery and additional endoscopic treatments on day 3 and day 21. However, the numbers of deaths by day 3, 21, or 35 were very similar. Conclusions: We found a beneficial effect of intravenous omeprazole in severe ulcer haemorrhage, with a reduction in the number of operations, in endoscopic treatments, and in the duration and severity of bleeding.