In duodenal ulcer studies comprising more than 2,000 patients the efficacy of omeprazole in doses of 20-40 mg once daily has been compared with the recommended doses of the H2-receptor antagonists cimetidine and ranitidine. At 2 weeks the differences in healing rates in favour of omeprazole were 12-33%; while they were somewhat lower at 4 weeks, ranging from 4 to 20%. In most studies the difference in healing rate was statistically significant. In some studies pain relief at 2 weeks was more pronounced on omeprazole. In three gastric ulcer studies omeprazole in doses of 20 or 40 mg once daily has been compared with ranitidine 150 mg twice daily. Healing rates on omeprazole were significantly greater in 2 of the 3 studies. Pain relief was faster on the higher dose of omeprazole. In prepyloric gastric ulcer the healing rate at 2 weeks was not significantly greater on 20 or 40 mg omeprazole than on ranitidine, while in another study the healing rate was significantly greater on 30 mg omeprazole as compared with cimetidine. In the first week of treatment more patients on omeprazole became free of pain in the latter study. The relapse rate after stopping therapy was similar for omeprazole and the H2-receptor antagonists independent of the type of ulcer. In conclusion, omeprazole heals ulcers faster than the recommended doses of H2-receptor antagonist. The difference in healing rate is more pronounced halfway than at the end of the treatment course.