Liver tissue of a newly diagnosed 30-year-old patient with abetalipoproteinemia was studied by light and electron microscopy. Despite accumulation of large quantities of lipid droplets in hepatocytes, the lobular architecture remained intact, and there was no fibrosis. Acanthocytes were readily identified in sinusoids by scanning electron microscopy. Profiles of rough endoplasmic reticulum, smooth endoplasmic reticulum and Golgi apparatus appeared normal by transmission electron microscopy. Lipid droplets were not bound to membranes and were not associated with the endoplasmic reticulum or Golgi apparatus. In abetalipoproteinemia, these morphological features are consistent with defective apolipoprotein B synthesis. This prevents assembly of lipoproteins that can be secreted through the normal intracellular communicating tubular network and, consequently, fat accumulates in the cytoplasm of the hepatocytes.