The authors describe the rare case of an infant, 35 days old at operation, with truncus arteriosus in which the right pulmonary artery rose from the left posterolateral side of the truncus arteriosus above the truncal valve, and the left pulmonary artery from the inferior aspect of the proximal third of the aortic arch. At operation, the right ventricle-pulmonary artery continuity was reconstructed with an aortic homograft. The left pulmonary artery was anastomosed "end-to-side" to the left carotid artery of the aortic homograft, and the right pulmonary artery "end-to-side" to the joined openings of the left subclavian artery and descending thoracic aorta. It was difficult to understand if the first tract of the left pulmonary artery (2 mm in diameter) was a true pulmonary artery, a ductus arteriosus or an anomalous vessel. The infant survived the operation. Intensive postoperative treatment was necessary to counteract severe episodes of pulmonary hypertension. Enoximone 5 mcg/Kg/min and prostacyclin 5-10 mcg/Kg/min were effective in reducing pulmonary vascular reactivity. Four month follow-up shows optimal surgical correction and normal hemodynamic findings.