Five cases are described in which noninflammatory pulmonary arterial aneurysmal disease existed. In each case, severe pulmonary hypertension had been present. In one (woman, age 50 years), the pulmonary hypertension was of the primary type, while in each of the others, congenital heart disease was an underlying condition (atrial septal defect in three cases; each woman, age range 35 to 56 years) and complete transposition in the remaining case (boy, age ten). In one case with atrial septal defect, saccular aneurysm involving each major pulmonary arterial branch was considered to have resulted from atherosclerotic disease. In each of the remaining cases, cystic medial necrosis was present, and the aneurysmal disease was the result of laceration of a major pulmonary arterial segment. In one of the cases with atrial septal defect, the laceration was recent and extended through the full thickness of the pulmonary trunk in a circumferential manner. In the remaining three cases, lacerations were usually old and had not extended through the full thickness of the involved arterial segments. Saccular aneurysm formation (usually multiple) had resulted from weakness of the wall as a result of laceration and retraction of tissue.