Aim. To study plasma cell-free DNA (pcfDNA) concentration and B-cell clonality in patients with diffuse large B-cell (DLBCL) and B-cell high-grade lymphomas prior to and at different stages of chemotherapy as well as the correlation between the data obtained and clinical and laboratory parameters. Materials & Methods. The study enrolled 23 DLBCL patients and 7 healthy donors (HD). Plasma was prepared from whole blood by centrifugation, pcfDNA was isolated with the commercial kit Qiagen (Germany). The concentration of pcfDNA was determined using fluorometer Qubit (USA). В-cell clonality was estimated by immunoglobulin gene analysis (BIOMED-2 protocol) in the tumor tissue and bone marrow core biopsy specimens obtained on diagnosis date as well as in the pcfDNA at 5 end points: prior to chemotherapy and after cycles 1, 2, 3, and 4. Results. Prior to therapy, all DLBCL patients showed significantly higher pcfDNA concentration than HD. Immunochemotherapy cycle 1 resulted in considerable increase in pcfDNA concentration. After cycle 2 and subsequent cycles, pcfDNA concentration gradually decreased. After cycle 4, the mean pcfDNA concentration was comparable with that of HD. In 95 % of patients В-cell clonality in pcfDNA corresponded to that identified in the tumor specimen. After immunochemotherapy cycle 1, В-cell clonality was detected in 50 % of patients, after cycle 2 it was shown by 15 %. Only 1 female patient retained В-cell clonality after therapy cycles 3 and 4. In HD, no В-cell clonality in pcfDNA was identified. Prior to therapy, the analysis revealed no correlation of either pcfDNA concentration or В-cell clonality in pcfDNA with age, sex, tumor spread, presence or absence of extranodal lesions, proliferation index Ki-67, and lactate dehydrogenase concentration. Conclusion. In patients with malignant hematological tumors, pcfDNA seems to be an interesting, easily accessible biological material deserving further investigation. Any studies of pcfDNA require long-term dynamical analysis and standardized methods of collection, storage and processing of the data obtained. In the long run, with more and more information, pcfDNA can become an important diagnostic marker of tumor heterogeneity and a reliable relapse predictor.