Abstract The congenital medial lesion of sortic coarctation was studied by light and electron microscopy. In aortas of young infants prominent morphological features of this lesion were interstitial edema, increased ground substance and proliferation of poorly differentiated smooth muscle cells. The latter contained large aggregates of glycogen granules and frequent profiles of dilated endoplasmic reticulum. Characteristic changes were observed in aortic coarctation with age. Thus, there was progressive accumulation of fibroelastic tissue in the extracellular space. Moreover, autophagic vacuoles, suggestive of focal cytoplasmic degradation, appeared in smooth muscle cells, which also revealed a gradual decrease in their glycogen content. Subsequently, there were frequent vacuolated areas in the cytoplasm which contained thin myofilaments associated with numerous clusters of ribosome particles. Elastic elements often showed evidence of degeneration in coarctations obtained from children older than 5 years. At this time, smooth muscle cells appeared well differentiated and contained scanty ergastoplasmic membranes. These observations suggest that the morphogenesis of arterial fibroelastosis is interrelated with various phases of cytodifferentiation. It is further suggested that aging of the arterial tissue is accelerated in areas of structural disorganization.