A case of hypobetalipoproteinemia is described; a 16-year-old girl had been suffering for nearly 2 years from diffuse abdominal pain. The only clinical features were liver steatosis, slightly increased amino transferases and an incipient polyneuropathy. No sign of malabsorption or gastrointestinal disease was found. She had extremely low levels of cholesterol and triacylglycerol in her serum, slightly decreased serum phospholipids and normal HDL-cholesterol levels. Apolipoprotein B-100 was approx. 8% of normal, whereas B-48 was present at essentially normal levels. Electron microscopy of lipoprotein particles showed normal morphology of LDL. Examination of close relatives showed no abnormalities. Southern blots revealed no major deletions or rearrangements at the genomic level. Although rare, a- and hypobetalipoproteinemia should be considered as possible etiologies in patients with unexplained steatosis in the liver.