OBJECTIVE: To find out if cimetidine has an immunomodulating effect on gastric carcinogenesis in rats. DESIGN: Experimental prospective study. SETTING: Teaching hospital, Norway. ANIMALS: 132 male PGV/Mol rats given gastrojejunostomies. INTERVENTION: Half the rats were given cimetidine in their drinking water postoperatively for a minimum of 38 weeks. They were killed after 52 weeks observation and the stomach was investigated macroscopically and microscopically. MAIN OUTCOME MEASURE: In the cimetidine fed group 19/48 animals developed cancer (49%), versus 12/43 (28%) in the control group (p = 0.24). CONCLUSION: Cimetidine had no immunomodulatory effect on the development of gastric cancer in rats.