The inhibitory effects of intravenous lansoprazole on gastric acid hypersecretion caused by stress after surgical invasion were studied in patients who underwent surgery under general anesthesia for gastrointestinal diseases. Patients in this study received either 15 mg or 30 mg of lansoprazole by slow intravenous injection twice daily for 3 days (six injections). The total number of patients studied was 82, with 41 patients in each group. Intragastric pH was measured every 8 h after the start of administration, and the inhibitory effects of intravenous lansoprazole on postoperative gastric acid secretion were evaluated using four grades: excellent, good, fair, and poor. In the 15-mg group, 65.8% (25 of 38 patients) were classified as good or excellent, and in the 30-mg group, 75.7% (28 of 37 patients) were good or excellent. No adverse effects directly attributable to the administration of lansoprazole were observed.