The hemostatic effect of cimetidine has been investigated in a double-blind, randomized trial in patients bleeding from acute mucosal lesions. During a 48-hr period, hemostasis was achieved in 5 of 15 patients (33%) treated with intravenous cimetidine administered with antacid therapy and in four of 12 patients (33%) treated with antacids alone. In a second stage of the trial, the effect of oral cimetidine in preventing further bleeding was investigated. Bleeding did not recur in 18 patients receiving cimetidine and antacids while 6 out of 20 patients who were given antacids alone suffered further hemorrhage during the 14-day period of the study. It is concluded that intravenous cimetidine administered with antacids was not found more effective in obtaining hemostasis than antacids alone. When given orally to patients who had been bleeding from acute lesions but were not doing so at the beginning of treatment, cimetidine with antacids is more effective than antacids alone in the prevention of further bleeding.