Introduction. The most frequently diagnosed mediastinal lymphomas include: Nodular Sclerosis Classical Hodgkin Lymphoma (NSCHL) and Primary Mediastinal Large B-cell Lymphoma (PMBL). In the new, 4th edition of the WHO Classification of Tumors of Hematopoietic and Lymphoid Tissue of 2008, a new category was created: “B-cell Lymphoma, Unclassifiable, with Features Intermediate Between Diffuse Large B-cell Lymphoma and Classical Hodgkin Lymphoma”. It has also been referred to as “Mediastinal Grey Zone Lymphoma”. Aim. The aim of this paper was to analyze morphological and phenotypic characteristics of three diagnostically difficult cases of mediastinal and lymph nodes lymphomas. Materials and methods. Immunohistochemical analysis was performed in two stages: 1) LCA, CD20, CD3, CD30, CD15, Ki67; 2) the panel was extended to include: antibodies Bcl2, Bcl6, CD10, MUM1, CD23, Fascin, transcription factors – PAX5, Oct2, BOB1; LCA/CD45, CD20, CD30, CD15, CD3, CD23. Case studies. The examination encompassed: 2 cases that demonstrated a discordance between the morphology and the phenotype, and 1 case in which two apparently independent neoplastic growths – PMBL and NSCHL – were diagnosed within 4 months. Patients: 2 women (22 and 31 years old) and 1 man (27 years old) – presented large mediastinal masses of diameter larger than 10 cm. Discussion. Differential diagnosis between NSCHL and PMBL is sometimes very difficult. However, NSCHL and PMBL demand different therapeutic strategies. In the case of PMBL treatment is more intensive. Thus, unambiguous diagnosis is necessary: either NSCHL or PMBL. In some cases, diagnostic difficulties may occur, sometimes it is even impossible to establish diagnosis. Conclusions. Among B-cell lymphomas in mediastinal tumors there are cases of untypical clinical course and untypical morphological and phenotypic characteristics. Thus, it is necessary to re-examine recurrences, including localizations other than the primary one. An adequate, i.e. large enough, specimen taken during mediastinoscopy is the basis for the correct diagnosis. In diagnostically complicated cases, it is necessary to extend the immunohistochemistry panel to include: CD23 and transcription factors: PAX5, Oct2 and BOB1.