The presence of peptic ulcer disease implies a high risk of bleeding in patients on heparin therapy. We reviewed our experience with 166 consecutive patients admitted because of venous thromboembolism. Of these 166 patients, 29 were referred for upper gastrointestinal endoscopy in order to detect the presence of any lesion that might contraindicate heparin therapy. A gastric ulcer was found in 10 patients, a duodenal ulcer in 11, and gastric erosions with signs of bleeding in 3 patients. Given the unexpectedly high frequency of ulcer in these patients, an upper gastrointestinal endoscopy was routinely performed early in the course of admission in 50 consecutive patients with venous thromboembolism. A gastric ulcer was found in 5 patients (10%), a duodenal ulcer in 7 (14%), and erosions in 2. Five of these patients had an unsuspected ulcer. A case can be made for prophylactic antiulcer therapy for all patients placed on anticoagulants for venous thromboembolism. Upper gastrointestinal endoscopy is indicated in patients with ulcer symptoms, in those with a previous history of peptic ulcer disease, and perhaps, in patients developing occult blood in the stools while on treatment with anticoagulants.