Serum gastrin was determined in 33 patients during treatment with the proton pump inhibitor omeprazole. After 4 weeks of therapy, gastrin levels increased to a median of 55 pg/ml compared to 15 pg/ml prior to omeprazole (P less than 0.001). There was a close correlation (r = 0.939; P less than 0.001) between pre-treatment gastrin and levels at 4 weeks. Comparison of serum gastrin concentrations at 1 month of omeprazole with levels at 6 (n = 21) and 12 months (n = 12) continuous therapy revealed a close correlation (r = 0.961 and r = 0.882, respectively; P less than 0.001) despite dose adjustment. In marked hypochlorhydria documented by continuous pH monitoring, serum gastrin varied from normal up to profound hypergastrinaemia. These results demonstrate that the serum gastrin increase under powerful acid-inhibitory drug therapy depends upon a number of variables. (a) Only in patients with elevated gastrin levels, prior to omeprazole treatment, can moderate to marked hypergastrinaemia during omeprazole be expected. (b) Gastrin increases reached during the initial period of omeprazole treatment remain constant during long-term therapy. (c) Acid inhibition itself is not necessarily associated with an increase in serum gastrin in every patient, which suggests that the individual sensitivity of the gastrin cell to acid inhibition is more important for serum gastrin changes than the degree of acid inhibition itself.