We report a case with late onset riboflavin-responsive multiple acyl-CoA dehydrogenation deficiency (MADD) characterized by decreased acyl-carnitine profile in serum which is consistent with primary systemic carnitine deficiency (CDSP) while just the contrary to a typical MADD. This patient complained with muscle weakness, muscle pain and intermittent vomiting, and was diagnosed as polymyositis, received prednisone therapy before consulted with us. Muscle biopsy revealed mild lipid storage. The findings of serum acyl-carnitines were consistent with CDSP manifesting as decreased free and total carnitines in serum. But oral L-carnitine supplementation was not very effective to this patient and mutation analysis of the SLC22A5 gene for CDSP was normal. Later, another acyl-carnitine analysis revealed a typical MADD profile in serum, which was characterized by increased multiple acyl-carnitines. Compound heterozygous mutations were identified in electron transferring-flavoprotein dehydrogenase (ETFDH) gene which confirmed the diagnosis of MADD. After administration of riboflavin, he improved dramatically, both clinically and biochemically. Thus, late onset riboflavin-responsive MADD should be included in the differential diagnosis for adult carnitine deficiency.