Primary mediastinal large B-cell lymphoma (PMBCL) is a distinct type of large B-cell lymphoma. In this type of the disease, the neoplastic process is located in the anterior and superior mediastinum, frequently with compression of the superior vena cava and with tumor invasion into the adjacent organs and tissues: the pericardium, lung, pleura, etc. Despite the fact that in PMBCL progression, there may be involvement of extranodal organs, such as the kidney, adrenal glands, liver, and central nervous system, bone marrow (BM) injury is generally absent. Since BM injury in patients with diffuse large B-cell lymphoma is an independent poor prognostic indicator, there is reason to believe that BM involvement in PMBCL affects the prognosis. These cases may need intensified induction therapy followed by autologous hematopoietic stem cell transplantation; and BM injury should be monitored during the therapy. The paper gives reports of clinical cases of bone marrow involvement in 2 PMBCL patients treated at the National Research Center for Hematology, Ministry of Health of the Russian Federation.