Dendritic cells (DC) are important antigen-presenting cells (APC) of immune system that induce and modulate primary and secondary immune responses; they also induce of central and peripheral tolerance T lymphocytes to self-antigens. They interact with T, B and NK cells and promote activation, differentiation and effectory functions of these cells. The total number of 91 patients: 53 males and 38 females, aged from 1 month to 18 years (median 9 years) treated in the Department of Children Bone Marrow Transplantation, Oncology and Hematology, Medical University of Wroclaw, Poland (ALL--39, AML--15, NHL--8, HD--10, LCAL--4, solid tumors--15 children) were examined. Peripheral blood was collected at time of diagnosis, during chemotherapy, at time of relapse or progression disease and analyzed with used flow cyto-metric analysis. The subpopulations of CD were examined with use of specific markers for distinct blood dendritic cell populations: BDCA1 BDCA2, BDCA3 (Miltenyi Biotec). Both of activated DC CD83+ HLA-DR+ and lymphocytes T CD3+CD26+ were stained of monoclonal antibodies CD83 (Pharmingen), CD11c, HLA-DR, CD3, CD26, CD19 (BD Bisciencies). The results of our study showed increased level of blood DC in ALL and AML patients at time of diagnosis, relapse or during progressive disease in comparison with control group of healthy children, as well as in other patients. Those yields explained positive correlation with value of activated lymphocytes T (p < 0.05).