The pulmonary perfusion of 25 children who had total surgical correction of tetralogy of Fallot was evaluated by radionuclide perfusion scans. In addition, 18 had 133Xe ventilation studies. Eighteen of the children previously had palliative systemic-pulmonary shunts; 14 had aortic-pulmonary shunts (Waterston or Potts-Smith) and four had Blalock-Taussig shunts. Seven children had single stage total corrections. An asymmetric perfusion pattern was found in 13 of 18 children who previously had systemic-pulmonary shunts, including 12 of 14 with previous aortic-pulmonary shunts (P less than 0.05). The most common finding was relative hypoperfusion of the lung which had received the palliative shunt (P less than 0.001). The distribution of ventilation remained relatively symmetric, even when perfusion was markedly abnormal, and this resulted in ventilation-perfusion imbalance in several patients. Asymmetric perfusion was significantly less common in patients who had undergone single stage corrections (P less than 0.05). The findings document the frequent occurrence of residual abnormalities of pulmonary perfusion and ventilation-perfusion imbalance in patients who have palliative aortic-pulmonary shunts prior to total repair, and support the position that single stage correction is preferable to aortic-pulmonary shunting in the surgical management of tetralogy of Fallot.