Familial defective apolipoprotein B-100 (FDB) is a recently identified dominantly inherited genetic disorder, which leads to increased serum levels of low density lipoprotein (LDL) cholesterol with reduced affinity for the LDL receptor. This genetic disorder is characterized by defective binding of the apolipoprotein B-100 (apo B-100), which is virtually the sole protein constituent of LDL, to the LDL receptor. The defective binding results from a G to A mutation at amino acid 10,708 in exon 26 of the apolipoprotein B (apo B) gene creating a substitution of glutamine for arginine in the codon for amino acid 3500. It is postulated that FDB can exhibit the same clinical features as familial hypercholesterolemia (FH) caused by a defective LDL receptor. The purpose of this paper is to report on an individual with a defective LDL and a defective LDL receptor. The clinical features of this individual were the same as in the family members with either defective LDL or a defective LDL receptor: premature arcus lipoides, tendon xanthomata, and premature atherosclerosis. Although the clinical features were present to the same degree as in individuals with either defect the prognosis and treatment of such an individual could be different.