MR blood velocity measurements were performed by the RACE technique in a plane perpendicular to the flow of the pulmonary arteries. MR findings were correlated with those of perfusion scintigraphy, Doppler US and right heart catheter (thermodilution). The ratio of MR blood flow measurements of right and left pulmonary arteries correlated well with the results of perfusion scintigraphy (RPA to LPA) and Doppler. Poor correlation was found when comparing MR blood flow measurements with right heart catheter since absolute flow measurements can be superimposed by neighboring blood vessels in complex anatomic situations.