In 12 patients with atrial septal defect or anomalous pulmonary venous connection, pulmonary vein wedge mean pressure uniformly exceeded mean pressure in the left atrium or pulmonary vein and with one exception was less than mean pressure in the pulmonary artery. There was a positive correlation between pulmonary arteriolar resistance and the difference in pressure between pulmonary artery and pulmonary vein wedge position. The oxygen saturation of blood, withdrawn from the pulmonary vein wedge position, uniformly exceeded that of left atrial and usually also that of pulmonary vein blood.