We studied ventilator circuit colonization and condensate formation in 30 mechanical ventilators during the first 24 h after a circuit change. Parts of the circuit nearest the patient were more frequently contaminated and had the highest levels of colonization. There was rapid colonization of tubing after a circuit change; 33% of the ventilators were colonized at 2 h, 64% at 12 h, and 80% at 24 h. The median level of colonization at 24 h was 7 X 10(4) organisms/ml. Water condensate collected in the ventilator circuits at a mean rate of 30 ml/h (range, 10 to 60 ml/h). At 24 h, 80% of the condensate samples were contaminated at a median level of 2 X 10(5) organisms/ml. The bacteria isolated from the condensate usually correlated with organisms previously isolated from the patient's sputum, suggesting that the patient's oropharyngeal flora is the primary source of circuit colonization. Highly contaminated condensate in the ventilator circuit may be a significant risk factor for nosocomial pneumonia. We suggest that circuit condensate be emptied regularly, handled as infectious waste, and that special efforts be taken to prevent contaminated condensate from inadvertently washing into the patient's tracheobronchial tree.