The impact of metabolic diseases (inborn errors of metabolism) and endocrine disorders in pediatrics has markedly increased during the last few decades. Critical periods in the development of the central nervous system need special attention in children with these disorders. Early diagnosis and treatment are important in order to prevent mental retardation and serious handicaps in some of these patients. Certain patients with metabolic and endocrine disorders lack early clinical symptoms or have so non-specific signs that permanent neurological handicaps are present when the patients are finally diagnosed. One way to identify these patients is by means of mass screening. A blood sample is then collected from every newborn infant and analyzed for abnormal levels of metabolites or hormones. It is possible to detect at least thirty different disorders in this way. In most European countries screening programmes involve phenylketonuria (PKU) and congenital hypothyroidism. The prognosis for these patients has improved dramatically after the introduction of screening. The Swedish neonatal metabolic screening programme was started in 1965 by screening for PKU. Subsequently, screening for galactosemia and congenital hypothyroidism was added. The result of the screening programme 1965-1985 is as follows: (table; see text) The main benefit of early detection and treatment of children with PKU, congenital hypothyroidism and galactosemia is the prevention of mental retardation and other handicaps. Recently nationwide pilot screening for congenital adrenal hyperplasia (adrenogenital syndrome) was started.