Omeprazole blocks the action of H+,K+-ATPase in the gastric mucosa and thus inhibits the secretion of hydrochloric acid. We conducted a double-blind multicenter study (45 centers in 13 countries) of 602 patients with benign gastric or prepyloric ulcers to compare the effectiveness of omeprazole (20 mg once daily, 203 patients, or 40 mg once daily, 194 patients) and ranitidine, an H2-receptor antagonist (150 mg twice daily, 205 patients) in promoting ulcer healing and to evaluate the pattern of ulcer relapse during a six-month follow-up. Healing occurred at four weeks in 80 percent of the patients receiving 40 mg of omeprazole, 69 percent of those receiving 20 mg of omeprazole, 69 percent of those receiving ranitidine. At eight weeks, the corresponding figures were 96, 89, and 85 percent. A multivariate analysis of ulcer healing showed that at four weeks the ulcers of significantly more patients receiving omeprazole had healed as compared with patients receiving ranitidine (omeprazole, 40 mg, vs. ranitidine, P less than 0.0005; omeprazole, 20 mg, vs. ranitidine, P = 0.01). At eight weeks, the 40-mg dose of omeprazole was significantly more effective than ranitidine (P = 0.001) or the 20-mg dose of omeprazole (P = 0.03). Ulcer symptoms were relieved faster with omeprazole. In 68 patients receiving concurrent nonsteroidal antiinflammatory drugs, the healing rates at four weeks were 81 percent in the group receiving 40 mg of omeprazole, 61 percent in the group receiving 20 mg, and 32 percent in the group receiving ranitidine; at eight weeks, the corresponding figures were 95, 82, and 53 percent. During the six-month follow-up period (without treatment), significantly more patients in the omeprazole groups were free of symptoms and ulcers than in the ranitidine group. We conclude that in the dose used, omeprazole is superior to ranitidine in the treatment of benign gastric ulcers.