In nine patients with pulmonary atresia, ventricular septal defect, and major aortopulmonary collateral arteries, the effect of a systemic-pulmonary anastomosis on the lung was studied by comparison of pre- and postoperative cine angiograms. Selective injections into the collateral arteries were performed in all patients and the source of blood supply to each bronchopulmonary segment was sought. Central pulmonary arteries were present in six children and absent in three, as confirmed at thoracotomy. After insertion of a shunt, central pulmonary arteries increased in size, but the intrapulmonary vessels with which they connected remained abnormally small and were frequently stenosed. In addition, in each case the central pulmonary arteries increased flow to only five to 11 segments of lung. In the absence of central pulmonary arteries, shunting to a hilar or a lobar pulmonary artery increased perfusion to five to nine segments of lung per case. In all these three cases the pulmonary arteries at lobar level showed aneurysmal dilatation proximal to a severe stenosis. Sixty-eight per cent of collateral arteries were stenosed. The findings suggest that in this anomaly, when the central pulmonary arteries are hypoplastic, the intrapulmonary branches are also hypoplastic, emphasising the need for early surgical intervention to increase blood flow while the lung still has growth potential. Further, one collateral artery may connect with at least as many bronchopulmonary segments as does a central pulmonary artery, and the peripheral intrapulmonary arteries with which it connects may appear at least as normal angiographically as do vessels connected to central pulmonary arteries. These observations suggest that segments of lung connected to collaterals alone should, and sometimes can, be connected to central pulmonary arteries, the aim being to produce a unifocal blood supply as a prelude to total correction.