We conducted a case-control study at Group Health Cooperative of Puget Sound to investigate the relation between long-term cimetidine use and gastric cancer. Five of 99 cases of gastric cancer were exposed to cimetidine at least two years before diagnosis, whereas nine of 365 controls (selected from among users of the Group Health Cooperative pharmacy, matched on age, sex, and first year of pharmacy use) were similarly exposed. The odds ratio comparing users with nonusers was 2.3 (95% CI = 0.8-6.9). The odds ratio comparing users of antacids with nonusers was similar, 1.9 (95% CI = 1.0-3.7). Although a causal relation between gastric cancer and cimetidine is possible, the similarity of the findings for cimetidine and antacids lends support to other explanations.