In 60 patients in whom Swan-Ganz catheters apparently had been positioned correctly, the balloon was visualized by inflation with radiopaque contrast medium. Sixteen were located peripherrally; in 15 of these 16, the balloon inflated eccentrically and in each of these instances, an accurate wedge pressure could not be obtained. One patient in this group had an episode of hemoptysis immediately prior to detection of the peripheral location and eccentric inflation of the balloon. The correct placement and safe use of the Swan-Ganz catheter demand that the catheter tip be located in a large pulmonary artery and that redundant loops of catheter be avoided to prevent subsequent peripheral migration. Identification of peripheral placement and eccentric inflation should be suspected if a pulmonary wedge pressure is obtained with a significantly smaller volume of air than the balloon capacity. The use of a continuous flush system will provide an additional alert by a steady rise in the pseudowedge pressure on attempted balloon inflation.