Familial defective apolipoprotein B-100 (FDB) and familial hypercholesterolemia (FH) are the common causes of monogenic primary hypercholesterolemia. An individual of mixed English and Afrikaner descent with both FDB and the FH Afrikaner-1 low-density lipoprotein receptor mutation was identified in our laboratory. Subsequent analysis of her extended family revealed the presence of heterozygotes for either FH Afrikaner-1, FH Afrikaner-2, or FDB as well as five additional double heterozygotes for FH Afrikaner-1 and FDB and one "complex" heterozygote with all three mutations. The hypercholesterolemic and clinical features of the pure FDB subjects were similar to those of the pure FH heterozygotes. The double heterozygotes with both FH and FDB have lipid levels and clinical features that are intermediate in severity between heterozygous and homozygous FH.