Omeprazole was administered for up to 6 years in 859 patients for 'prevention of relapse in patients with poorly responsive peptic ulcer or reflux oesophagitis'. The pattern of adverse events reported during long-term treatment was similar to the adverse-event profile in short-term treatment with omeprazole (n = 2,818), ranitidine (n = 1,572) and cimetidine (n = 891). Omeprazole had essentially the same adverse-event profile as the two H2-receptor antagonists. The adverse-event profile for omeprazole during long-term treatment did not differ from that seen during short-term treatment with either omeprazole or the H2-receptor antagonists. The rate of occurrence of any specific adverse event decreased with time, and no previously unidentified adverse event occurred with long-term omeprazole therapy. There were no serious adverse events that were considered to be causally related to omeprazole therapy. Thus, omeprazole has been shown to be well tolerated in both short- and long-term treatment.