Electron microscopic investigations were performed on liver biopsies of 24 patients with endogenous hyperlipoproteinaemia (HLP) of the types IIa--V, classified by lipid- and lipoprotein-fractions, as to Frederickson et al. (4), as well as on biopsy specimens of 6 control persons without histological alterations, and on 7 biopsies with staetosis hepatis. In type IIa we found a typical proliferation of the smooth endoplasmic reticulum and a slight alteration of the mitochondria. Types IIb--V are characterized by increasing fatty degeneration of the liver cells as well as by quantitative and qualitative alteration of the endoplasmic reticulum and the Golgi areas. Giant mitochondria, organelles with atypical membrane structures and paracrystalline matrix inclusions occur more frequently, especially in patients with additional diabetes mellitus. There are scarcely differences between steatosis hepatis without HLP and steatosis with combined HLP, resp., except the more distinct alterations of mitochondria, endoplasmic reticulum, and Golgi areas in the latter. The morphological changes are especially restricted to those liver cell structures, involved in the metabolic disregulation.