Familial combined hyperlipidemia (FCHL) is the most common genetic form of hyperlipidemia in which affected individuals manifest multiple lipoprotein phenotypes. Although the molecular defect is still unknown, several kinetic studies have demonstrated increased turnover rates of apolipoprotein B (apo B) in patients with FCHL, irrespective of their lipoprotein phenotype. Using 3 restriction fragment length polymorphisms (RFLPs) of the apo B gene (XbaI, MspI and EcoRI) we have investigated 33 families which fulfill the diagnostic criteria of FCHL. No significant difference in allele frequency was found between 33 unrelated individuals with FCHL and 107 normolipidemic controls. 3-RFLP haplotypes were constructed in each pedigree. A co-segregation analysis was performed in 7 informative families. In no family was co-segregation observed between the haplotype of the apo B gene and the phenotype of FCHL. These data are not compatible with the hypothesis that FCHL is caused by mutations of the apo B gene acting as a simple mendelian trait.