Inborn errors of metabolism (IEM) are not have specific clinical signs, they masquerade as other diseases, and are difficult to diagnose using only clinical manifestations or routine laboratory tests. IEM most commonly manifest in early infancy and childhood. Despite the fact that most IEM are rare in the population, they occupy one of the first places in the structure of childhood pathology, early infant mortality and disability. IEM often remains undiagnosed, while timely diagnosis and timely treatment started can prevent severe systemic damage leading to death and disability. The appointment of a special treatment (diet therapy, cofactors, enzyme replacement therapy) prevents or significantly inhibits the development of the pathological process, especially if the diagnosis is made in the early stages of the disease. To start pathogenetic treatment as early as possible, it is necessary to diagnose IEM as accurately and as early as possible. Among the diseases included in mass screening programs IEM are especially important due to the development of disability and early mortality in the absence of timely diagnosis and treatment, as well as a high risk of recurrence in burdened families. In this connection, the main goals of mass screening - the prevention of disability in children and the reduction of early infant mortality - dictate the need to introduce modern technologies for preclinical diagnosis of IEM. Based on the results of the study, it is planned to scientifically substantiate the need for the introduction of selective screening of children for hereditary metabolic diseases using the technology of tandem mass spectrometry in the Republic of Kazakhstan for timely diagnosis, therapy of IEM and prevention of disability. The introduction of a selective newborn screening program for IEM should always be preceded by a study aimed at studying the prevalence of the disease in a certain region, determining regional reference values of the studied metabolites. Local incidence and outcome data can be used to persuade health officials to prioritize screening in health care spending. The main scientific question and hypothesis of the project is whether it is necessary to introduce tandem mass spectrometry technology in the neonatal screening program for IEM.