A four-month-old child with non-ketotic hypoglycemia and rapidly progressive cirrhosis excreted in her urine large amounts of two unidentified organic acids in addition to a spectrum of saturated, unsaturated, and 3-hydroxy dicarboxylic acids in her urine. Gas chromatography/mass spectrometry of the trimethylsilyl derivative of one of the unknown compounds suggested the structure of 3-hydroxyoctanoic acid, which was confirmed by similar analysis of the authentic compound. The same organic acid was found in the child's plasma. The significance of 3-hydroxyoctanoic aciduria as a possible marker for a primary defect of 3-hydroxy fatty acid metabolism is discussed.