We have investigated the family of a 15-year-old proposita with a homozygous, receptor-defective, familial hypercholesterolemia and found that her consanguineous, obligate heterozygous parents have "normal" cholesterol levels and a family history of unusual longevity. Documentation of paternity and the presence of the heterozygous biochemical disorder in the parents is firm. The implications are that, at least in this family, relatively low serum cholesterol and high levels of HDL cholesterol are protective against the risks associated with having a mutant allele for heterozygous familial hypercholesterolemia.