The clinical features of eight patients with tetralogy of Fallot and major aortopulmonary collateral arteries were determined and found to be usually unhelpful in differentiating such patients from those with pulmonary atresia with ventricular septal defect. The point of connection between the pulmonary and collateral arteries could usually be demonstrated at cineangiography by observing washin and washout between the two systems. The central pulmonary arteries connected with the entire pulmonary vascular bed in the five patients in whom the anatomy of the pulmonary circulation was clearly demonstrated and were thought to do so in the remaining three patients also. In six out of eight patients only one collateral artery connected with each hilar pulmonary artery. These findings suggest that the anatomy of the pulmonary circulation in patients with tetralogy of Fallot and major aortopulmonary collateral arteries is usually less complex than in pulmonary atresia with ventricular septal defect, making the outlook for complete repair more hopeful. The major determinant of successful complete repair appears to be pulmonary arterial size. Early palliative surgery to increase pulmonary blood flow is recommended to encourage normal growth of the central and intrapulmonary arteries.