Abstract —More than 90% of patients with type III hyperlipoproteinemia are homozygous carriers of the apolipoprotein (apo) E*2 allele. The great majority of these apoE2(Arg158→Cys) homozygotes in the general population, however, are normolipidemic. Apparently, expression of the hyperlipidemic state requires additional genetic and/or environmental factors, suggesting a multifactorial etiology. To elucidate these additional risk factors, we analyzed normolipidemic and hyperlipidemic apoE2 homozygotes. Hyperinsulinemia was observed in 27 of 49 apoE2 homozygotes and associated with elevated lipid levels: hyperinsulinemic apoE2 homozygotes had type III hyperlipoproteinemia 6 times more often than apoE2 homozygotes with normal insulin levels (odds ratio 6.2, P =0.02). We screened the normolipidemic and hyperlipidemic apoE2 homozygotes for common variants in candidate genes involved in lipolysis—the APOA1-C3-A4 gene cluster, lipoprotein lipase, and hepatic lipase—and analyzed for associations with the expression of hyperlipidemia. In the hyperinsulinemic group, the 7 carriers of the Sst I polymorphism (S2) in the APOC3 gene displayed severely elevated VLDL cholesterol ( P _{insulin by Sst I} <0.001) and VLDL triglyceride ( P _{insulin by Sst I} <0.01) and low levels of HDL ( P _{insulin by Sst I} <0.02). In the normoinsulinemic group, no such relation of the Sst I polymorphism with hyperlipidemia was observed. These data provide the first evidence for a combined effect of hyperinsulinemia and the Sst I polymorphism on the expression of hyperlipidemia in apoE2 homozygotes.