The objectives of this study were to determine the optimal duration of acid suppression required for the initial treatment of gastric ulcer, and to compare the acid-suppressive effects of omeprazole with the H2-blocker cimetidine in patients with gastric ulcer as determined by 24-hour measurement of intragastric pH. Cimetidine was administered daily to 31 patients, while omeprazole was administered daily to 16 patients and every second day to an additional 16 patients. Ulcers healed within 2 months of treatment in 16 of the 31 patients administered cimetidine. The pH 4 holding time (the proportion of the total 24-hour period of measurement during which intragastric pH was 4 or higher) was 51.9 ± 4.6% of the 24-hour period of measurement for these 16 patients. Patients with unhealed ulcers had significantly shorter durations of ‘pH 4 holding time’ (15.8 ± 2.9% of the 24-hour period). There was no difference in the ‘pH 4 holding time’ of the 24-hour period between those receiving cimetidine who had a curable ulcer within 2 months and those administered omeprazole on alternate days (57.5 ± 7.9%). These findings suggest that obtaining the optimal duration of acid suppression is necessary for the treatment of gastric ulcers. Omeprazole administered as an alternate-day regimen is useful not only for the prevention of recurrence, but also for the initial treatment of gastric ulcer.