Since the cloning of the human LDL receptor (LDLR) gene, familial hypercholesterolemia (FH) can be diagnosed by recombinant DNA technology either using restriction enzyme mapping to detect major rearrangements of the gene or using restriction fragment length polymorphisms (RFLPs) and linkage analysis in family studies. Genotypes and haplotypes of four RFLPs (StuI, ApaII 5', PvuII, NcoI) were used to study the inheritance of the detective LDLR gene in three families. Diagnosis of FH based on the lipid levels alone was not possible because in these kindreds both parents exhibit elevated lipid levels. However, in two families using haplotype analysis, elevated cholesterol levels in certain relatives could be attributed to the inheritance of a defective LDRL gene and thereby distinguished from hypercholesterolemia due to familial combined hyperlipidemia. In the third family where both hypercholesterolemic parents carried a defective LDLR gene, a case of homozygous FH could be excluded in a child by demonstrating the inheritance of a normal LDLR gene.