Retrospective analysis of the course of 125 patients in whom flow-directed balloon-tipped catheters were employed showed that in nine (7.2 per cent), pulmonary ischemic lesions appeared to have occurred as a direct result of the use of the catheter. Mechanisms of the pulmonary ischemia included persistent wedging of the catheter tip in a peripheral artery (six cases), obstruction of a more central pulmonary artery by an inflated balloon (one case), and pulmonary embolism from venous thrombosis developing around the catheter (two cases). Awareness of the tendency for spontaneous wedging of the catheter to occur and of the possibility of air remaining in the balloon after use should reduce the frequency of these complications. (N Engl J Med 290:927–931, 1974). THE introduction, in 1970, of the flow-directed balloon-tipped catheter (Swan–Ganz catheter) for the study of pulmonary capillary wedge pressures1 has greatly facilitated hemodynamic investigations of severely ill patients. Reports on the use of these catheters have listed few complications attributable to them. These have included minor atrial and ventricular arrhythmias, rupture of the balloon without serious sequelae,2 intracardiac knotting of the catheter3 and rupture of a pulmonary artery.4 5 6 Swan et al.1 described a case in which a thrombus developed in the superior vena cava and enveloped the shaft of the catheter. Recent reviews of the use of the flow-directed balloon-tipped.