The aim of this study is to evaluate the therapeutic effect of proton pump inhibitors on peptic ulcers resistant to H2-receptor antagonists. Patients with ulcers resistant to at least 3 months treatment with standard or greater doses of H2-receptor antagonists were treated with 20 mg of omeprazole or 30 mg of lansoprazole, once daily, for 2 to 8 weeks. Endoscopy was performed every 2 weeks to confirm ulcer healing. Eleven of 28 (39%) gastric ulcers healed within 4 weeks and 20 (71%) within 8 weeks with omeprazole. Eight of 19 (42%) gastric ulcers healed within 4 weeks and 14 (74%) within 8 weeks with lansoprazole. All of the duodenal ulcer healed within 6 weeks with omeprazole or lansoprazole. No adverse effects were observed in this study. These results suggest that proton pump inhibitors are highly effective in the treatment of peptic ulcer resistant to H2-receptor antagonists.