Blood gas values were obtained from Swan-Ganz pulmonary artery catheters in 25 patients with acute pulmonary failure, with the objective of evaluating the possibility of contamination with “arterialized” blood and examining the mechanism by which this might happen. Blood oxygen content increased significantly from the main to a segmental pulmonary artery, proportional to the withdrawal rate of the sample. At 3 ml/min, distal contents ranged from 100 to 116% of proximal values (p > 0.01). At 23 ml/min the range was 100–140% of proximal values (p > 0.001). Sampling of blood from a Swan-Ganz catheter in the usual position for “wedge” pressure measurement, but with balloon deflated, may lead to large errors in calculation of cardiac output by the Fick method and in calculation of intrapulmonary shunt fraction.