In a double-blind, dose-comparative trial, 32 patients with duodenal ulcer were assigned to receive either 20 mg/day or 60 mg/day omeprazole for 4 weeks. The 2-week healing frequency of 100% in the 60 mg/day group was significantly higher than that in the 20 mg/day group (63%). After 4 weeks all ulcers but one in the 20 mg/day group were healed (93% healing frequency). In both groups transient and mostly slight rises in serum alanine aminotransferase levels were observed (total 10 patients). One patient in the 20 mg/day group was withdrawn because of a pronounced rise in serum alanine aminotransferase on day 8. The reason for these liver reactions is not clear, but exclusion of a causal relation with omeprazole treatment must precede further clinical evaluation of this drug.