Bacteriologic cultures were performed on the tips of pulmonary artery catheters removed from 153 critically ill patients, who had required pulmonary artery catheterization for management of hypovolemic or septicemic shock or for hemodynamic monitoring during mechanical ventilation with positive end-expiratory pressure. Positive results were obtained in 29 (19%) of the cases. Infection of indwelling pulmonary artery catheters may result from contamination during placement or removal or from transient or persistent bacteremia. Colonization was probable in 17 cases, and contamination in 12. There were no instances of sepsis definitely attributable to the catheter. Positive catheter-tip culture was associated significantly with known presence of a focus of infection before catheter insertion and with periods exceeding four days that the catheter remained in place.