We report a patient who had childhood onset medium-chain acyl Co-A dehydrogenase deficiency, confirmed by gas chromatography-mass spectrometry (Gc-Ms). A 9 year 3 month old girl had suffered from generalized weakness and hypotonia, including vomiting, lethargy and comatous consciousness.

The autosomal-recessively inherited myopathy is characterized by organic aciduria that distinguishes from Reye syndrome and other masqueraders characterized by dicarboxylic aciduria.

The key metabolities, subglycine and hexanoylglycine, are excreted in high concentration only when the patients are acutely ill. More significantly, using novel technique in Gc-Ms, the medium-chain acylcarnitines, mostly octanoylcarbnitine are excreted without significant excretion of a normal metabolite when the patients are not ill. EMG findings usually show "myopathic" features.

In view of the carnitine deficiency and the demonstration of excretion of the toxic medium-chain acyl Co-A compounds as acylcarnitines, a combined therapy of reduced dietary fat, 50∼150 mg/day of riboflavin and L-carnitine supplementation (25 mg/kg/6h) has been devised and applied with positive outcome in our patient.