A randomized, controlled clinical trial was performed to compare the efficacy of prophylactic cimetidine treatment, antacid treatment and no treatment in high risk patients. Sixty patients received cimetidine (200 mg every 6 hours), 52 patients received antacids (Maalox, 10 ml/hour), and 56 patients received no treatment. One hundred thirty-five patients completed the study. Eight patients in the control group and one patient in the antacid-treated group presented with upper gastrointestinal bleeding. No bleeding occurred in the cimetidine-treated group. Patients considered at low risk had no bleeding. Cimetidine or antacid prophylaxis significantly decreased the incidence of upper gastrointestinal bleeding in the high risk patients.