Seoul Clinical Laboratories began screening newborns and high risk group blood spots with tandem mass spectrometry (MS/MS) in April 2001. The goal was to determine approximate prevalence of metabolic disorders and optimization of decision criteria for estimation of preventive effect with early diagnosis. Approximately 44,300 neonates and children were screened and the estimated prevalence (newborn/high risk group), sensitivity, specificity and recall rate amounted to 1:2000 / 1:1250, 94.1 %, 99.7 %, and 0.04 %, respectively. Confirmed 35 multiple metabolic disorders (newborn/high risk) were as follows; 16 amino acid disorders [classical PKU(3/4), BH4 deficient-hyperphenylalaninemia(0/1), Citrullinemia(2/0), Homocystinuria(0/2), Hypermethioninemia(0/1), Tyrosinemia(1/0)], OTC deficiency (0/1), MSUD (2/0), 10 organic acidurias [Propionic aciduria(2/1), Methylmalonic aciduria(0/1), Isovaleric aciduria(2/1), 3-methylcrotonylglycineuria(1/0), Glutaric aciduria type 1(2/0)], 9 fatty acid oxidation disorders [LCHAD def. (2/2), Mitochondrial TFP def.(0/1), VLCAD def.(1/0), LC3KT def.(0/1), SCAD def (1/0), MADD def (0/1). The relatively normal development of 15 patients with metabolic disorders among newborns (except for the expired) demonstrates the usefulness of newborn screening by MS/MS for early diagnosis and medical intervention. However, close coordination between the MS/MS screening laboratory and the metabolic clinic/biochemical geneticists is needed to determine proper decision of screening parameters, confirmation diagnosis, follow-up scheme and additional tests.