At autopsy, two infants had unsuspected coarctation of the left pulmonary artery (CoLPA), which was produced by an extension of ductal tissue into the wall of the left pulmonary artery. The first case, a 4-month-old girl, also had a ventricular septal defect and an anomalous branching pattern of the innominate arterial trunk. Pulmonary arterial hypertensive changes were noted in the right lung. In contrast, the left lung showed thin-walled pulmonary arteries. The second case, a term female newborn, had exhibited severe unexplained respiratory distress since birth. Histologic sections of the right lung showed dilated pulmonary arteries with thinned media, whereas the left lung showed a persistent fetal arterial pattern. It is believed that the peripheral pulmonary arterial changes are age-dependent and associated with asymmetric blood flow between the right and left pulmonary arteries. CoLPA is a rare pulmonary artery defect, and early diagnosis of this abnormality is important.