Lipoprotein composition varies among different genetic forms of hyperlipidemia. An increase in hepatic triglyceride (TG) synthesis in subjects with familial hypertriglyceridemia (FHTG) is associated with secretion of large, TG-enriched, very low-density lipoproteins (VLDL), which have an increased affinity for lipoprotein lipase (LPL) in vivo as compared with VLDL from subjects with familial combined hyperlipidemia (FCHL) or from normal subjects. Elevated levels of plasma low-density lipoprotein (LDL) apoprotein B in FCHL are associated with high apoprotein B production rates. The LDL in FCHL is heterogeneous, with a preponderance of an LDL subfraction, which is denser, smaller, and lipid poor as compared with LDL from normal subjects. The more buoyant LDL subfraction in FCHL seems to be catabolized more rapidly than this dense LDL subfraction.