Аннотация:The known prognosis factors for rhabdomyosarcoma (RMS) in childrendo not always explain the unsatisfactory outcome of treatment. Changes inthe subpopulation composition of Bone Marrow (BM) effector cells duringthe development of RMS may indicate new directions for the search for prognosticfactors and points for the impact of targeted therapy. Purpose: To identifycorrelations between quantitative changes in the levels of subpopulationsof T, B and NK-lymphocytes of BM and known risk factors for RMS in children.Objects: The study included 31 patients. The main group included 16 patientswith RMS, average age—6.8 ± 1.0 years, while children 1 - 10 yearsold—13 (81.3%), over 10 years old—3 (18.8%) people, girls and boys were 8people each. The embryonic variant of RMS was established in 10 (62.5%) cases,the alveolar variant—in 4 (25%) cases, in two patients (12.5%), the histologicalvariant was not established. In 12 (75%) patients, an unfavorable localizationof the RMS (parameningeal, extremities, prostate, bladder) was revealed,in 4 patients (25%), the localization of the tumor was regarded as favorable.Patients with T2b—13 (81.2%) and T2a—2 (12.5%) stages prevailed. Regionaland distant metastases were detected in 10 (52.6%) patients. The comparisongroup included 15 children in whom the presence of malignant neoplasia wasexcluded, the average age was 8.4 ± 1.5 years, 11 boys (73.7%) and 4 girls(26.3%). Methods: All patients underwent morphological (myelogram counting)and immunological (quantitative analysis of lymphocytic subpopulations)bone marrow studies. Immunophenotyping in all patients was carried out bydirect immunofluorescence using a triple fluorescent label. Results: Significantdifferences in the levels of subpopulations of BM T-lymphocytes werefound when comparing the values of the main group, distributed by localizationand histological variant, with the data obtained in the control group ofpatients. For example, the percentage of CD3+ T cells with the co-stimulatorymolecule CD28+ was significantly higher in patients with parameningeal RMS(p = 0.010). Conclusion: Each clinical group of patients has its own individualimmunological characteristics. The results obtained by us can beconsidered indicative and regarded as starting points for further study ofthe peculiarities of the subpopulation composition of BM in patients withRMS.