Multiple acyl-CoA dehydrogenase deficiency is a rare autosomal recessive inborn error of metabolism. The late-onset multiple acyl-CoA dehydrogenase deficiency is frequently caused by mutations in ETFDH gene. Because of its clinical heterogeneity, diagnosis and treatment of late-onset multiple acyl-CoA dehydrogenase deficiency are often delayed. The authors described a previously healthy 40-yr-old Thai woman presenting with subacute severe weakness of bulbar-limb muscles and elevated serum creatine kinase. The authors emphasized the importance of needle EMG and prompt muscle histopathological evaluation, which rapidly led to the diagnosis and riboflavin therapy, resulting in a dramatic and rapid improvement before genetic study disclosed mutation in ETFDH gene.