A total of 143 patients with peptic ulceration of the duodenum, stomach or oesophagus, who did not respond to 3 or more months high-dose treatment with ranitidine (450 mg or more daily), were admitted to oral treatment with omeprazole, 40 mg/day. In 94.4% of the patients, ulcers healed within 2-6 weeks. After healing of their ulcers, 122 patients were admitted to long-term maintenance treatment with omeprazole, 40 mg/day; 91 patients have been on the drug for 1-5.5 years. During maintenance therapy with omeprazole, 40 mg/day, no relapses (verified by endoscopy) have yet occurred and no drug related adverse effects have been observed. There were no significant changes in routine laboratory tests in any patients, including 27 with concomitant liver cirrhosis. Serum gastrin levels were already elevated approximately 2-fold during the initial high-dose ranitidine treatment and rose a further 2-fold at 2-3 months of omeprazole treatment. Thereafter, no further increase of serum gastrin was observed even after 5.5 years of continuous observation. Volume density of G and D cells in the antral mucosa did not change significantly. The volume density of argyrophilic cells in the oxyntic mucosa was 0.73 +/- 0.1% before the start of omeprazole treatment and 0.85 +/- 0.09% after 17-24 months of continuous treatment with omeprazole (ns). In antrectomized patients the volume density was lower (0.23 +/- 0.04%). No clusters of argyrophilic cells were observed in any of the groups. In a control group of patients with gastrinoma the volume density of these cells was higher (1.3 +/- 0.23%, n = 8).(ABSTRACT TRUNCATED AT 250 WORDS)