We tested the influence of the apolipoprotein E (apoE) polymorphism on the intrapair differences in the levels of plasma cholesterol, plasma triglycerides, low density lipoprotein-cholesterol, apoB and apoE in monozygotic (MZ) twins, and estimated whether or not there was a interaction between the apoE polymorphism and environmental factors. In 65 MZ twin pairs, the intrapair differences in the measured lipoprotein parameters were similar in the different apoE phenotype classes. This indicates that the effect of the apoE polymorphism is not influenced by environmental variability between the MZ pair members and accordingly identifies the APOE gene as a level gene.