Relevance. Children with congenital heart disease (CHD) are at high risk of infectious complications with unfavorable outcomes, which is associated with inadequate immune responses. TREC and KREC are recognized biomarkers of T- and B-cell lymphopoiesis.Objective of the study: to evaluate the number of circular DNA segments — TREC and KREC in children with congenital heart defects.Materials and methods. The study used data from neonatal screening for primary immunodeficiencies in the Stavropol region from January 1, 2023, to June 30, 2024. An analysis of TREC and KREC was conducted in 43 newborns with CHD compared to healthy infants.Results. The levels of TREC and KREC in children with CHD were lower than in healthy children. A correlation was established between TREC levels and absolute lymphopenia, as well as the development of infectious complications. A decrease in KREC was identified in infants with CHD and genetic syndromes. It was shown that a TREC level of less than 650 copies per 105 cells may be a predictor of the development of infectious complications in newborns with CHD.Conclusions. The reduction in the number of TREC and KREC copies in neonatal screening allows for the identification of children with CHD at high risk of infection, opening potential opportunities for preventive therapy.