Twenty-four children, aged 4 months to 16 years (nine patients 2 years old or younger), underwent balloon dilation angioplasty of hypoplastic or stenotic branch pulmonary arteries between July, 1981, and April, 1984. Most children had tetralogy of Fallot, with or without pulmonary atresia or isolated peripheral pulmonary artery stenosis. Fifty-two dilations were attempted, 44 in the catheterization laboratory and eight in the operating room. Of these, 26 (50%) were judged successful; the average vessel diameter on angiogram increased from 4.1 ± 0.3 to 7.2 ± 0.3 mm (76%), the gradient across the narrowed segment fell from 60 ± 10 to 36 ± 5 mm (40%), pressure in the main pulmonary artery or right ventricle proximal to the obstruction decreased from 83 ± 10 to 66 ± 6 mm Hg (20%), and the radionuclide-determined fraction of cardiac output directed to the lung ipsilateral to the dilated pulmonary artery increased from 40 ± 4 to 51 ± 4 (28%). All changes were significant at the p < 0.005 level. Reasons for failure included inadequate technique (balloon too small, inability to position balloon or wire) in 14 and refractory nature of the lesion itself in 11. Technical failures were age independent. Nondilatable lesions were more common in children more than 2 years old (10/25 versus 1/10) or with isolated peripheral pulmonary artery stenosis (5/7). Five of seven stenoses near previous shunts were nondilatable. One child exsanguinated when the pulmonary artery ruptured during dilation, but the other complications were few. Eight dilations, followed up for an average of 6 months after dilation, showed angiographic persistence of improvement; two of four lesions were successfully redilated to a larger size. Balloon dilation angioplasty appears beneficial, both short and long term, for some patients with hypoplastic or stenotic branch pulmonary arteries, especially if performed early in life.