Pulmonary artery catheters (PAC) inserted using pressure monitoring usually advance into a branch of the right pulmonary artery (PA). However, in some clinical situations it may be desirable to locate the tip of the PAC in a branch of the left PA. A two-part study was undertaken to explore factors that determine the ultimate location of the tip of a PAC. In the clinical study, 33 patients were randomized to supine or right-side down patient positioning during insertion of the PAC. Five patients were excluded because of withdrawal of the PAC during surgery. Seven of 14 PACs inserted in right-side down position were located in a branch of the left PA, while all 14 inserted with the patients supine went to a branch of the right PA (P less than 0.005). This suggests that the effect of upward flotation of the air-filled balloon in the column of blood predominates over movement with the current of maximal blood flow. Using a cardiac bypass pump and a plexiglass model of the pulmonary arterial bifurcation, the effects of lateral positioning on the direction of travel of balloon-tipped catheters were studied. The tip floated upward preferentially, even when flow was obstructed to the upward limb (P less than 0.005). Varying the flow rate in the system between 0.5 and 3.0 L/min and changing the direction of the natural curve of the PAC tip had no impact on this tendency to float upward. The authors conclude that balloon flotation in the column of blood strongly influences PAC tip location, and that this fact can assist in preferentially directing its placement.