In 1949, Hellems, Haynes, and Dexter proposed that the pressure in a catheter wedged so as to occlude a pulmonary artery was an "estimate" of the pressure in the pulmonary capillaries. Their report led to the designation of this pulmonary artery wedge pressure as the pulmonary "capillary" wedge pressure. In fact, the pulmonary artery wedge pressure is a distorted measure of the pressure in the pulmonary veins. Usually this pressure differs only slightly from the capillary pressure, and the misconception fostered by the inaccurate name is inconsequential; however, sometimes this misconception leads to errors in diagnosis. This report briefly reviews the early history of pulmonary vascular catheterization, offers an explanation of pulmonary artery wedge pressure, and discusses a disease in which pulmonary artery wedge pressure is normal, even though capillary pressure is elevated.