To establish the sources of lymphocytosis in children living in ecologically unfavorable conditions after the Chornobyl Nuclear Power Plant accident aiming the high risk group formation for oncohematological diseases among the children's population.The 226 children with lymphocytosis were surveyed. The of child's life history and kind of somatic morbidity were considered. Quantitative and qualitative parameters of blood cells, biochemical indices including immunoglobulins (A, M, G), circulating immune complexes, phagocytosis indices were assayed.Children with lymphocytosis were more often born with a high bodyweight, they demonstrated manifestations of constitutional lymphatism, lymphadenopathy and more often had respiratory infections. Quantitative and qualitative changes of hemopoietic elements depended on a type of a disease. Activation of lipid peroxidation, dysimmunoglobulinemia and increased level of circulating immune complexes were revealed. Treatment and preventive actions promoted normalization of hemogram indices in 58% of children surveyed.The abnormalities in immune indices, activated lipid peroxidation in membranes of hemopoietic elements, and manifestations of granulocytopoietic disorders were established it children with lymphocytosis. Therefore these children can be attributed tot the group of increased risk on oncohematological disease.