THE UNDERSTANDING of the etiology of atherosclerosis has been a goal of many medical research workers for many years. Various theories (i.e., filtration, mechanical factors, endothelialization of mural thrombi, and derangement of the metabolism of the aortic wall itself) have been proposed by investigators in the past, and no doubt all of the factors can be implicated as contributing to the pathological condition that the clinician sees in his office. It is our own feeling that the most important factor in the etiology of atherosclerosis, is the participation of the arterial wall itself in atheroma formation. The arterial wall can be considered a complex organ carrying on all of its metabolic functions and at the same time withstanding the constant pressure of the blood that traverses it. Detailed studies of the lipoid content (we include cholesterol, phospholipid, fatty acids, and fat in this term) of the initial lesion have shown