Abstract The frequency of familial hypercholesterolemia (FH) in the Province of Quebec is twice that observed in most population samples. In the French-Canadian population, 5 mutations in the low density lipoprotein (LDL) receptor gene account for 76% of FH cases diagnosed using clinical and biochemical criteria. One of these mutations, a >10 kilobase (kb) deletion at the 5′ end of the gene involving the promoter and exon 1, is present in 60% of FH patients studied in Montreal. This high frequency is attributable to a “founder effect.” Contribution to this founder effect included strategies in 17th century France for the settlement of a small number of pioneers, local incentives for early marriages and large families, and geographic and cultural isolation; these factors also favored endogamy. The > 10 kb deletion is present on only one haplotype in all patients studied so far (homozygotes and heterozygotes), a major advantage for studies relating phenotypic variation to haplotype variation in the normal LDL receptor allete of these hemizygous patients. Indeed, the presence of founder effects facilitates screening, genetic counseling, and treatment. It also confers a number of advantages for research: (1) the search for new causes of FH in the 24% unaccounted for to date; (2) the study of the geographic distribution of mutations and population movements; (3) the identification of gene-gene interactions in the etiology of disease; (4) the evaluation of factors modulating phenotypic expression and search for LDL-lowering genes; and (5) the study of genetic determinants of therapeutic response.