OBJECTIVE: To find out whether patients with acute bleeding peptic ulcers have hypercoagulable blood. DESIGN: Prospective open study. SETTING: District hospital in Sweden. SUBJECTS: 54 consecutive patients with duodenal ulcer (n = 25) and gastric ulcer (n = 29) admitted with haematemesis or melaena, or both. INTERVENTIONS: Diagnosis verified on admission by endoscopy, and healing was examined at follow up. Consumption of non-steroidal anti-inflammatory drugs, and smoking habits, were recorded. MAIN OUTCOME MEASURES: Coagulation of the blood monitored by concentrations of plasma thrombin-antithrombin III (TAT) complex in samples obtained on admission for the acute bleeding episode and at follow up 1-2 months later. RESULTS: The plasma TAT complex concentrations were raised during the acute bleeding episode (p