In order to evaluate the incidence and the significance of complications resulting from the use of flow--directed, balloon--tipped catheters to monitor critically ill patients, we made a prospective study of 116 pulmonary artery catheterizations. Indications for catheterization included shock, pulmonary edema, or hemodynamic instability following surgery. Arrhythmias, including premature atrial or ventricular depolarizations, ventricular tachycardia, and transient right--bundle branch block occurred during 90 of the 116 insertion procedures, but were unassociated with morbidity or mortality. In two cases (1.7 percent) staphylococcal bacteremia probably originated from the catheter. In addition, the pulmonary artery catheter led to two cases (1.7 percent) of subclavian vein thrombosis. Postmortem examinations revealed perforations of the pulmonic valve in one case. We conclude that although significant complications may result from pulmonary arterial catheterization and monitoring of critically ill patients, the incidence is low.