Bronchoalveolar lavage (BAL) has been proposed as a useful procedure for bacteriologic diagnosis of lower respiratory tract infection in mechanically ventilated patients. To determine the cardiopulmonary effects of this procedure and to identify the patients at risk of poor tolerance, 30 critically ill ventilated patients suspected of having pneumonia were studied. Hemodynamic and gas exchange parameters were continuously recorded using an arterial catheter, a Swan-Ganz catheter with SvO2 display, and a pulse oximeter. In addition to the basal sedation required by these patients, midazolam, 0.1 mg/kg intravenously, was administered 5 min prior to bronchoscopy. A moderate increase (10 percent from basal values) in heart rate, mean arterial pressure, and cardiac index was recorded at each measurement during the procedure. A marked decrease in PaO2 was observed during bronchoscopy associated with an increase in oxygen consumption. Maximal changes in SaO2 and SvO2 were recorded at the end of BAL. Two hours after the end of BAL, PaO2 values were still 20 percent lower than pre-BAL values in 40 percent of the patients. We conclude that BAL can be performed safely in most critically ill ventilated patients who have stable hemodynamic and ventilatory parameters. However, none of the recorded parameters allows identification of the patients at risk of poor tolerance of the procedure.