Consecutive patients with overt signs of upper gastrointestinal bleeding will be randomised to receive omeprazole or its equivalent placebo (80mg i.v. bolus followed by 8mg/hr) until next scheduled endoscopy. Patients with circulatory instability or fresh hematemesis will undergo endoscopic examinations urgently after initial stabilisation. The infusion will continue until endoscopic examination with or without therapy. Next scheduled endoscopy is performed within 24 hours from admission by experienced endoscopists with expertise in therapeutic endoscopy. At endoscopy, actively bleeding ulcers or ulcers with non-bleeding visible vessels are to be treated by endoscopic therapy. Omeprazole infusion 8mg/hr is to be continued for 72 hours after endoscopic hemostasis.