In pulmonary atresia with ventricular septal defect and major aortopulmonary collateral arteries, there is a tendency for each collateral artery to be the sole supply to a particular region of lung. On injection into a collateral artery, however, "wash-out" of contrast medium by non-opacified blood from a second source is sometimes seen. Alternatively, contrast medium may faintly "wash-in" to an adjacent lobar artery supplied from a second source. "Wash-in" and "wash-out" therefore show that a duplicate blood supply exists. To assess the importance of these phenomena, we reviewed the angiograms of 50 patients with this condition. Duplicate pulmonary blood supply not resulting from surgical shunts was found in 25 patients (50%), and in 24 of 37 patients (65%) who had selective collateral injections. In one patient two collaterals anastomosed with each other. Thirty-nine instances of duplicate supply occurred in the remaining 24. The duplicate connection was to a region of lung connected to a central pulmonary artery in 29 of 39 instances (74%). Determination of how much of the peripheral pulmonary vasculature is connected to central pulmonary arteries greatly affects decisions about palliative and corrective surgery. These results show that unless "wash-in" and "wash-out" are specifically looked for, the information obtained from angiography may be erroneous.