The application of combined gas-liquid chromatography mass spectrometry in the analysis of physiological fluids from patients in whom a metabolic disorder is suspected has revealed many new inborn errors of metabolism, especially organic acidurias. Inborn errors have been identified in the conversion of cholesterol into steroid hormones and bile acids (for review see ref. 1). To our knowledge no inborn error of cholesterol biosynthesis has been reported. In view of the important function of cholesterol with respect to synthesis of biolo~cally active steroid compounds and membrane function we wish to report a patient with a persistent urinary excretion of mevalonic acid, a precursor in the synthesis of sterols.