The medium-chain acyl-coA dehydrogenase deficiency is one of several metabolic disorders presenting clinically as Reye syndrome. Evidence is presented for a characteristic organic aciduria that distinguishes this disorder from Reye syndrome and other masqueraders characterized by dicarboxylic aciduria. The key metabolites, suberylglycine and hexanoylglycine, are excreted in high concentration only when the patients are acutely ill. More significantly, using novel techniques in mass spectrometry, the medium-chain defect is shown to be characterized by excretion of specific medium-chain acylcarnitines, mostly octanoylcarnitine, without significant excretion of a normal metabolite, acetylcarnitine, in four patients with documented enzyme deficiency. Similar studies on the urine of two patients reported with Reye-like syndromes of unidentified etiology have suggested the retrospective diagnosis of medium-chain acyl-coA dehydrogenase deficiency. Administration of L-carnitine to medium-chain acyl-coA dehydrogenase deficiency patients resulted in the enhanced excretion of medium-chain acylcarnitines. Octanoylcarnitine is prominent in the urine both prior to and following L-carnitine supplementation. The detection of this metabolite as liberated octanoic acid, following ion-exchange chromatographic purification and mild alkaline hydrolysis, provides a straightforward diagnostic procedure for recognition of this disorder without subjecting patients to the significant risk of fasting. In view of the carnitine deficiency and the demonstrated ability to excrete the toxic mediumchain acyl-coA compounds as acylcarnitines, a combined therapy of reduced dietary fat and L-carnitine supplementation (25 mg/kg/6 h) has been devised and applied with positive outcome in two new cases.