&NA;Newborn screening for inborn errors of metabolism has improved the diagnosis and treatment of these disorders since the 1960s. Recently tandem mass spectrometry (MS/MS) was developed as a technique for expanding the scope and efficiency of newborn screening for inborn errors. It may offer more efficient identification of phenylketonuria, branched chain ketoaciduria (maple syrup urine disease) and homocystinuria, which are currently screened for by the use of bacterial inhibition assays. MS/MS also identifies analytes characteristic of disorders of fatty acid metabolism and organic acid metabolism, which are not identified in current programs. Recent studies indicate that MS/MS offers the opportunity to expand and advance newborn screening for inborn errors.