LDLs are lipoprotein complexes referred to as "bad cholesterol" because prolonged elevation of LDL levels leads to atherosclerosis. LDLs are the primary transport vehicle for cholesterol synthesized in the liver (See Figure 18.7). Oxidation of LDLs leads to foam cell formation and ultimately an atherosclerotic plaque. The primary apoprotein contained in LDLs is Apo B-100.

LDLs transport cholesterol into target cells by binding to an LDL receptor and being absorbed into the cell by a process referred to as receptor-mediated endocytosis. People with the hereditary disease familial hypercholesterolemia have elevated serum cholesterol levels arising from problems with their LDL Receptors.