Treatment of renal transplant patients with the H2-antagonist cimetidine was thought to be of reasonable prophylactic value in controlling the incidence of the postoperative complications of gastric or duodenal ulceration. It was attempted to evaluate performance of the drug in a controlled trial by treating transplant patients with cimetidine or a placebo. Of the 59 patients accepted for the trial, 4 had to be excluded eventually because of irregularities in the administration of the drug and, in 1 case, nonfatal respiratory failure. Of 27 from the cimetidine group 6 had erosions or ulcers by the 3rd day after surgery and 2 more had them by the end of the 4th week. Of 28 placebo patients, 3 developed lesions after 3 days and 3 more developed them after 7 wk. In the months after transplantation, 1 cimetidine and 2 placebo patients developed ulcers. Bleeding occurred 3 times with cimetidine and twice with the placebo. Renal function was similar in both groups as was the necessity of transplantectomy because of irreversible rejection. Cimetidine does not lower the incidence of gastroduodenal mucosal lesions and upper gastrointestinal bleeding after renal transplantation, nor does it influence rejection of the allograft.