Accelerated atherosclerosis occurs in patients with type III hyperlipoproteinemia and familial hypercholesterolemia. The accumulation of chylomicron remnants of intestinal origin and of VLDL remnants or IDL of hepatic origin observed in type III hyperlipoproteinemia appears to correlate with coronary disease. The presence of defective forms of Apo E prevents normal receptor-mediated catabolism of these lipoproteins. Patients with familial hypercholesterolemia have an elevation of plasma IDL secondary to defective LDL receptors that impair normal catabolism. Familial defective Apo B100 is secondary to an abnormality of Apo B100 that prevents the normal interaction of LDL with the LDL receptor and increases plasma LDL. Macrophages (which are derived from circulating monocytes) have emerged as a key component in atherogenesis because they appear to be progenitors of foam cells in arterial lesions. Macrophages express receptors that recognize chylomicron remnants and VLDL remnants and chemically modified LDL. Thus, in the presence of these specific lipoproteins, macrophages are converted to cells that resemble foam cells.