The lung was biopsied in 20 children with double inlet ventricle and pulmonary hypertension aged 2 months to 14 years. Eleven patients had two patent atrioventricular valves, three atresia of the right valve, and six hypoplasia of the left valve. Severe pulmonary arterial medial hypertrophy occurred in the nine children less than 1 year of age. The findings did not suggest a period of normality after birth when the pulmonary artery might have been banded most effectively. Of the 11 older patients, eight had medial hypertrophy and three intimal proliferation with medial atrophy. Six patients with medial hypertrophy had some reduction in pulmonary arterial pressure after banding. It is recommended that the pulmonary artery be banded as early as possible, and rebanded early if a satisfactory result is not obtained, particularly in patients destined for a Fontan-Kreutzer procedure. Early atrial septectomy should reduce the arterial and venous abnormalities seen in left atrioventricular valve hypoplasia.