The intimal thickness of the coronary arteries of humans less than 20 years of age was compared with that of 9 other species. It was found that humans were unique amongst the species studied in showing substantial intimal thickening at an early age. The intimal thickening was associated with defects of the internal elastic lamina (IEL), which in humans were not repaired, but which in other species were associated with an effective reduplication of the IEL. This reformed IEL appeared in other species to form a significant obstruction to the diffusion of macro-molecules from the lumen into the arterial wall, but did not do so in humans. Preliminary observations also indicated that the endothelial cells were closely associated with a well-formed elastin membrane in other species, and formed a continuous inner lining for the arterial wall. In human coronary arteries with substantial intimal thickening, the endothelium was incomplete, with many bare areas.