The development of elastin lamellae in the media of elastic type arteries such as the aorta appears to be modulated by mechanical factors such as flow. On that basis, decreased aortic flow during fetal development is held responsible for the occurrence of tubular hypoplasia of the aortic arch. Underdevelopment of the ascending aorta in aortic atresia likewise is considered due to diminished retrograde flow from the aortic arch. The present study has examined this hypothesis by comparing the number of elastic lamellae in hypoplastic segments of the aortic arch and ascending aorta with corresponding segments from normal aorta. In both situations the underdeveloped segments show a paucity of cellular and supportive connective tissue constituents, leading to a densely packed layer of elastic lamellae and, thus, contributing to the diminished dimensions. A major difference, however, was noted with respect to the number of elastic lamellae. In the hypoplastic ascending aorta in aortic atresia, the number was not different from that encountered in normal specimens. In tubular hypoplasia, on the other hand, the number of elastic lamellae was significantly lower than in the corresponding segments of normal specimens (P less than 0.01). Taking the stance that flow is a major factor in proper development of the aortic media, it seems that the findings in tubular hypoplasia fit well with a chronic lack of tension from early development and, hence, relate directly to the cardiac malformation. The findings in the ascending aorta of aortic atresia are less easy to understand from this point of view.(ABSTRACT TRUNCATED AT 250 WORDS)