The inhibitory effect of omeprazole on gastric acid secretion was tested in a group of patients on haemodialysis for chronic renal failure. Single 30 mg doses almost totally inhibited basal acid output on both dialysis and non-dialysis days. Plateau acid output was reduced by a mean of 77% and 90% on non-dialysis and dialysis days respectively. The absorption and pharmacokinetic profile of omeprazole were not affected by dialysis. Omeprazole was not recoverable from dialysis fluid. It is concluded that omeprazole is a potent inhibitor of gastric acid secretion in patients with chronic renal failure, and its effect is not influenced by haemodialysis.