Omeprazole efficacy and tolerance were evaluated in 20 patients with longstanding Zollinger-Ellison syndrome (ZES) committed to long-term antisecretory therapy. The study included 13 men and 7 women, aged 53 (30-74) years (median and range). Nineteen patients presented with epigastric pain, 14 with vomiting, and 9 with diarrhea. All patients had gastroduodenal ulcerations, associated with esophagitis in 9 cases. Median and extreme values for basal acid output (BAO) and serum gastrin (SG) levels before omeprazole treatment were 41 (3.7-80) mmol H+/h and 413 (111-11,490) pg/ml, respectively. In 18 patients, omeprazole treatment was initiated because of resistance to H2-antagonists, and in 2 patients because of carbothioamide RP 40749 discontinuation. Initial doses of omeprazole were 60 mg per day in 10 patients and ranged from 80 to 160 mg per day in the others. Esophagogastrectomy was performed in one patient at day 15 because of esophageal stenosis. In the remaining 19 patients, median duration of treatment was 16 (7-54) months and median doses of omeprazole were 70 (20-160) mg per day during the survey. Omeprazole therapy was highly effective in inducing rapid disappearance of clinical abnormalities in 18 of 19 patients. Twenty-two days after initiation of treatment, median BAO was 4 (0-14) mmol/h and ulcerations had healed in 17 of 19 patients. Median BAO was less than 5 mmol/h during follow-up. However, asymptomatic ulcer recurrence was noted in 4 patients, but disappeared quickly after omeprazole doses were increased. Median basal gastrin level was 700 (116-36.625) pg/ml at the least determination and was statistically higher than pretreatment values (p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)