BACKGROUND Treatment of omeprazole induces profound inhibition of gastric acid secretion, resulting in hypergastrinaemia. In rats hypergastrinaemia induced by chronic administration of high doses of omeprazole resulted in ECL-cell hyperplasia and subsequent carcinoid formation. This finding may limit long-term therapy in man. The synthetic prostaglandin E2 analogue enprostil not only inhibits gastric acid secretion but also reduces serum gastrin in normal subjects and in peptic ulcer patients. The present study was undertaken to determine whether enprostil reduces serum gastrin in patients on long-term treatment with omeprazole. METHODS Eight patients with reflux oesophagitis treated with 40 mg omeprazole once daily for at least 3 months received 35 micrograms enprostil t.d.s. during a 5-day treatment course. Basal and postprandial serum gastrin concentrations and pepsinogen A and C levels were measured on the day before, the first and the final day, and on the day after cessation of treatment. RESULTS Enprostil significantly (P < 0.05) reduced basal serum gastrin from 65 +/- 15 pmol/L to 51 +/- 13 pmol/L on the first treatment day, and to 41 +/- 9 pmol/L on the final day. Enprostil also significantly (P < 0.05) reduced postprandial integrated serum gastrin from 6173 +/- 849 pmol.h/L to 4516 +/- 906 pmol.h/L and to 3532 +/- 706 pmol.h/L on the first and final treatment days, respectively. On the day after cessation of treatment basal (57 +/- 11 pmol/L) and postprandial integrated serum gastrin concentrations (5766 +/- 864 pmol.h/L) were not significantly different when compared to pretreatment values. Enprostil had no significant influence on serum pepsinogens A and C. CONCLUSIONS Short-term co-administration of enprostil lowers the serum gastrin levels in patients on long-term treatment with omeprazole.