A total of 143 patients with peptic ulceration of the duodenum, stomach and oesophagus who did not respond to 3 or more months of high-dose treatment with ranitidine, 450 mg or more daily, were treated with oral omeprazole, 40 mg daily. In 94% of the patients, ulcers healed within 2–8 weeks. After healing, 133 patients underwent long-term maintenance treatment with omeprazole, 40 mg daily, for 1–5 years (continuing). During maintenance therapy with omeprazole, no endoscopically verified relapses occurred, and no drug-related adverse effects were seen. There were no significant changes in routine laboratory tests in any patient, including 27 with concomitant liver cirrhosis. Basal serum gastrin levels, which were already elevated by the previous high-dose ranitidine treatment, rose to 4 times normal levels after 4 months of treatment with omeprazole. Thereafter, no further increases in basal serum gastrin levels were observed, even after 5 years of administration. The volume density of argyrophilic cells in the oxyntic mucosa increased during omeprazole treatment but no dysplasia of the gastric enterochromaffin-like cells was seen. In conclusion, omeprazole was highly effective in healing ranitidine-resistant peptic ulcers, and subsequent maintenance therapy with omeprazole, 40 mg daily, was found to be effective and safe over the period observed.