The gastric and upper-airway flora of 60 consecutive patients treated with antacids or cimetidine in a respiratory/surgical intensive-therapy unit were studied. In 52 (87.0%) patients one or more organisms were cultured simultaneously from both upper airway and stomach. A sequence of transmission was clear in 17 of these patients. Pneumonia due to gram-negative bacilli developed in 31 patients; in most cases the causative organisms were of gastric origin. No pneumonia developed in the 8 patients whose gastric and upper-airway flora were different. The number of gram-negative bacilli in gastric aspirates correlated with the pH of the gastric aspirate. Treatment of seriously ill patients with antacids or cimetidine may encourage airway colonisation and predispose the patients to pneumonia caused by gram-negative bacilli.