Balloon dilation angioplasty (BDA) was attempted in seven children with either stenosis or hypoplasia (a narrowing greater than 1 cm long extending past the lung hilum) of both right and left pulmonary arteries (PAs). In two of these seven, the procedure could not be performed because of technical difficulties. In each of the remaining five children (1 1/2 -16 years old), the right ventricular (RV) pressure was greater than 2/3 left ventricular pressure, main PA pressure was greater than 60 mm Hg, and previous operative attempts to relieve RV outflow obstruction, including the branch PA obstruction, were unsuccessful. BDA was performed in only one PA in each patient, and was considered successful if the pressure gradient decreased, angiographic diameter increased and the percentage of blood flow directed to the dilated lung increased. BDA was successful in all five children: RV pressure fell from 104 ± 42 to 80 ± 30 mm Hg (p < 0.05), the gradient across the obstruction fell from 61 ± 51 to 32 ± 22 mm Hg (p < 0.05), the diameter of the narrowed segment increased from 3.7 ± 1.2 to 6.8 ± 1.1 mm, p = 0.02), and the percentage of blood flow (as determined by quantitative lung scan) to the dilated lung increased from 41 ± 16% to 52 ± 22% (p < 0.05). No morbidity was observed in any patient. Follow-up angiograms (2-12 months) in three of five patients indicate persistence of the anatomic improvement. While BDA did not restore right-heart pressures and anatomy to normal, it provided significant hemodynamic relief to a group of patients in whom traditional operative management has usually been unsuccessful. Final determination of the role of BDA in such patients must await the results of further studies.