The studies reported here, coupled with our previous studies, indicate that LDL is ingested by cultured human fibroblasts in a process that resembles adsorptive endocytosis. The critical step is the binding of the lipoprotein to a high-affinity cell surface receptor. Uptake of LDL by this receptor-mediated process permits the cell to aquire cholesterol from the lipoprotein, and this acquisition, in turn, suppresses the cell's own cholesterol synthesis and activates the cell's system for reesterification and storage of the incoming cholesterol. In cells from patients with the receptor-negative form of homozygous FH, the cell surface receptor is functionally absent. The absence of high-affinity binding to this receptor produces a defective uptake of LDL and prevents the normal process of feedback regulation of cholesterol synthesis by the lipoprotein. The physiologic importance of the LDL pathway is indicated by the fact that patients who lack the LDL receptor (FH homozygotes) develop both profound hyper-cholesterolemia and fulminant atherosclerosis. It is likely that other defects in the LDL pathway account for other forms of hypercholesterolemia and atherosclerosis in man.