Aspirin induces gastric mucosal damage and bleeding in the presence of acid. Cimetidine, the histamine H2-receptor antagonist, reduces basal and stimulated acid secretion. Arthritic patients taking fixed doses of aspirin who were found to have aspirin-induced occult gastrointestinal bleeding were given cimetidine in a randomized double blind, crossover study. Autologous 51Cr-labeled blood was measured in 4-day stool collections at the end of each 4-week period of placebo and cimetidine therapy in 22 acid-producing patients. Mean daily fecal blood loss was reduced during cimetidine therapy to 2.2 +/- 0.3 ml per day, compared with 4.1 +/- 0.7 ml per day during placebo therapy (P= 0.002).