With increasing age, the thoracic aorta shows progressive fibroplastic intimal thickening, which is thought to be pre-atheromatous. A similar progressive intimal thickening in the renal cortical arteries is the distinguishing feature of the nephrosclerosis which underlies essential hypertension. Therefore, the earliest detectable youthful precursors of atherosclerosis and hypertension show strong morphological resemblances to each other. In this study, close statistical associations have been shown between the two types of arterial intimal fibroplasia. Both conditions show similar sigmoid growth curves from ages 6 to 70 years, thereby generating correlations across age groups of r = 0.99 in New Orleans and r = 0.95 in Mexico City. Specimens gathered in New Orleans were found to have about 1.4 times greater arterial intimal thickening than specimens from Mexico City, and this excess was seen at all ages in both the aortas and the renal cortical arteries. It seems likely that intimal fibroplasia of arteries is reflecting similar biological principles at all levels of the vascular tree. Whatever etiological factors vary between New Orleans and Mexico City, those factors appear to act directly at a tissue level to promote the early precursors of atherosclerosis and of the nephrosclerosis that underlies hypertension.