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Background: Fatty acid oxidation abnormalities such as dicarboxylic aciduria (DCA) and increasedserum esterified carnitine (Cn) have been demonstrated in SMA children and may correlate with diseaseseverity due to denervation.Objective: To stratify SMA severity and to test whether increased serum esterified Cn, decreased serumfree Cn and abnormalities in serum acylcarnitine profile correlate with disease severity.Methods: 13 SMA patients (1 SMA I, 10 SMA II, 2 SMA III; 9F, 4M) and 12 age-matched axonalCharcot Marie Tooth (CMT) subjects (8F, 4M) were recruited. All underwent modified Hammersmithfunctional motor scale, neuromuscular disease QOL questionnaire and serum Cn (total, free),acylcarnitine profile and urine organic acids. SMA patients also had pulmonary functions and CMAPstudies. Age and sex-matched standardized control biochemical data were obtained. Comparisonsbetween groups were performed using t-test and ANOVA.Results: Median age of SMA was ~8 years (13 months -17 years). 6 SMA had reduced serum free Cnand 3 in total Cn. 2 CMTs had reduced serum free and total Cn. Acylcarnitine profiles were normal inall with the exception of increased C18:1 in one SMA II. One SMA I had mild DCA and small 3-methylglutaconic acid and 3-hydroxyisovaleric acid; 2 SMA II had small to moderate 3-hydroxyisovaleric and trace methylglutaconic acids. CMTs had normal organic acids. None hadincreased serum esterified Cn.Conclusions: A larger cohort will be needed to determine whether serum Cn profile may prove useful asa biomarker in SMA for disease severity and response to Nusinersen.