We obtained liver biopsies in 4 patients with type IIA and 7 patients with type IIB hyperlipoproteinemia, and also in 7 healthy controls. A morphometric analysis of liver tissue was carried out by light and electron microscopy. Structural alterations of the hepatocyte and its organelles in type IIA and IIB hyperlipoproteinemia seem to fall into different categories: smaller nuclei, enlarged peroxisomes and altered mitochondria are found in both types of hyperlipoproteinemia, while an increase in the volume of hepatocytes is found only in type IIA. The enlargement of liver peroxisomes in untreated hyperlipoproteinemia is noteworthy, since it has been considered as a marker of the liver toxicity of lipid-lowering drugs.