To prevent stress bleeding, 400 postoperative patients in intensive care but not expected to need long-term mechanical ventilation, were randomly given either 50 mg pirenzepine or 200 mg ranitidine daily intravenously for a mean of 3.9 days. Macroscopically visible bleeding was the criterion of stress bleeding. In addition, special attention was also paid to any signs of pneumonia. There was a significantly higher incidence of gastric pH values of less than 4 in the pirenzepine patients. Six episodes of bleeding occurred in the ranitidine group vs. three in the other. There was a significantly higher incidence of pneumonia among ventilated patients (18.0% vs. 2.7%). Among ventilated patients the pneumonia rate under ranitidine was 28.6% vs. 9.1% in the pirenzepine group (P less than 0.05). The probable cause of the higher pneumonia rate under ranitidine was the gastric colonization with gramnegative organisms. Pirenzepine assures an effective prophylaxis against stress bleeding at least as good as ranitidine. At the same time, the risk of lung infection is also lower with pirenzepine than ranitidine.