Description and discussion of the findings in a five month old infant and a six year old boy, in whom unilateral absence of the pulmonary artery contralateral to the aortic arch could be demonstrated by means of angiocardiography. The infant, having absence of the right pulmonary artery associated with patent ductus arteriosus and obstructive pulmonary vascular disease of the left lung died after recurrent pulmonary infections in acute heart failure. The boy with absence of the left pulmonary artery could be shown to have moderate-severe peripheral stenosis at the origin of the right pulmonary artery from the main pulmonary artery. Pulmonary function studies showed normal values. During a follow-up of 5 years the boy doesn't show any symptoms. In both cases an accessory vessel could be demonstrated on the side of the absent pulmonary artery originating from the innominate artery and leading to the hilum of the corresponding lung. Embryologically these vessels probably represent persistence of the second primitive ductus arteriosus. According to its development the disease more precisely should be designated as unilateral proximal aplasia of the 6th ventral aortic arch, i.e. of the definitive pulmonary artery.