Backgrounds: To confirm the diagnosis of cases showing elevated levels of leucine in blood, we developed a simple enzymatic diagnosis method for maple syrup urine disease which measures isovaleryl-CoA converted from 2-isoketocaproate by crude lysate of lymphocytes using HPLC (J Inherit Metab Dis 27, 2004) . Patients: We applied the enzymatic diagnosis method to subjects with 4 mg/dl or higher concentration of leucine; in dried blood spots of (A) 9 symptomatic and (B) 12 asymptomatic newborns, and in plasma of (C) 5 symptomatic children.