We surveyed bronchial microflora by alternate-day, non-directed bronchial lavage (NBL) in 150 patients requiring mechanical ventilation on an intensive care unit. This simple technique uses a 20 ml non-bronchoscopic lung lavage, then quantitative bacterial culture. NBL bacteriological findings were identical to those obtained by same-day bronchoscopic broncho-alveolar lavage on 16/20 occasions. Using serial NBLs, the bronchial bacterial population was characterized during 65 episodes of pneumonia defined by clinical and retrospective criteria. Mean bacterial colony counts increased significantly during the 2 days preceding the clinical onset of pneumonia, from < or = 10(3) cfu/ml to > or = 10(5) cfu/ml (p < 0.05). In 51 patients showing a clinical response to antibiotic treatment, mean colony counts fell significantly after antibiotic initiation (p < 0.05). By contrast, in 14 patients who showed progressive clinical deterioration or relapse, there was no significant fall in NBL counts, and serial NBLs revealed antibiotic resistance or superinfection. The surveillance data altered clinical management in 42% of patients. Positive NBLs guided the choice of antibiotics, whilst negative NBLs encouraged the withholding of antibiotics, or detection of alternative pathology. We propose routine bacteriological lung surveillance of mechanically ventilated patients using this simple, inexpensive and safe technique.