Autosomal recessive hypercholesterolemia (ARH) presents with a clinical phenotype similar to that of classical homozygous familial hypercholesterolemia (FH) caused by defects in the low-density lipoprotein (LDL) receptor gene but is more variable, generally less severe, and more responsive to lipid-lowering therapy than homozygous FH; furthermore, FH is inherited with a dominant pattern. The approximately 50 known affected ARH individuals are mostly of Sardinian or Middle Eastern origin, but rare cases of ARH have occurred worldwide. The physiological defect in ARH is a failure of some, but not all, cell types to mediate LDL receptor-dependent internalization of LDL and is caused by mutations in the gene for a putative adaptor protein called ARH. In affected cells, the LDL receptor gene is normal but LDL receptor protein accumulates at the cell surface; this also occurs in livers of recombinant mice lacking ARH, providing an explanation for the failure of clearance of LDL from plasma in ARH patients. The structural features of the ARH protein and its capacity to interact with the internalization sequence of the LDL receptor, plasma membrane phospholipids, and the clathrin endocytic machinery suggest that it plays a key role in the LDL receptor pathway.