Background-The importance of pulmonary regurgitation (PR) after repair of tetralogy of Fallot (TOF) is increasingly recognized, but little is known regarding its underlying mechanisms. This phase-contrast cine magnetic resonance (PC MR) study was performed to evaluate the relative contribution of each lung to total regurgitant volume. Methods and Results-Twenty-two patients with significant PR underwent a PC MR 3 to 16 years after repair of TOF.Regurgitant fraction of the main pulmonary artery was 39Ϯ10%. Regurgitant fraction of the left pulmonary artery (LPA; 46Ϯ18%) was greater than that of the right pulmonary artery (34Ϯ16%; Pϭ0.002). The average contribution of the LPA to the total regurgitant flow volume was 54Ϯ19%, whereas its average contribution to the total forward flow volume was 44Ϯ13% (Pϭ0.002). In 4 patients, regurgitant flow in the LPA accounted for 75% to 100% of the total regurgitant flow. There was a linear relationship between regurgitant fraction and fraction of the regurgitant flow duration in the main pulmonary artery (PϽ0.001) and right pulmonary artery (Pϭ0.001) but not in the LPA (Pϭ0.129). Conclusions-PR after repair of TOF is commonly associated with differential regurgitation in the branch pulmonary arteries, which is usually greater in the LPA. Although the cause of this disparity requires further investigation, those patients with a significant unilateral contribution to total PR may be amenable to localized techniques to reduce regurgitation. (Circulation. 2003;107:2938-2943.)