Introduction. Thrombocytosis is an increase of the number of platelets in peripheral blood more than 400 thousand/ml. Thrombocytosis can be used as an important prognostic factor of survival for patients with breast cancer, ovarian cancer and other. The aim. To evaluate the relationship between the overall 10-year survival of patients with and without thrombocytosis in patients with primary B-cell mediastinal lymphoma (PMBCL). Materials and methods. The study included 40 patients with PMBCL, who received treatment in the department of oncohematology of the Federal State Budgetary Institution "National Medical Research Centre for Oncology" of the Ministry of Health of the Russian Federation. Results. Initial thrombocytosis was observed in 23 patients. The median follow-up was 29.2 months. The overall survival in patients with baseline thrombocytosis was 39,847±7,102 and 33.0±11,468, while the overall survival without changes was 34,923±7,029 and 35.0±8,388 (Chi-squared 8,530, p ther changes in the hematopoietic system, but requires further research.