Diagnostically primary central nervous system lymphomas (PCNSL) have to be differentiated from glioblastoma and brain metastases. Histologically the overwhelming majority of PCNSL is represented by diffuse large B-cell lymphomas, in this series with a BCL6 expression in 80% of the cases detected by immunohistochemistry. Stereotactic biopsy is the method of choice in establishing the definite diagnosis and intraoperative smear cytology will detect the lymphoid blasts. To confirm the B-cell lineage, immunohistochemistry is needed (CD20, CD79a). Small reactive T-lymphocytes and monohistiocytic cells and activated "microglia" are found within and at the periphery of PCNSL foci. The infiltrated brain tissue shows partially pleomorphic reactive astrocytes that can be confused with neoplastic astrocytes, especially in small specimens. In contrast to high-grade gliomas, however, PCNSLs do not show endothelial proliferations. Subtypes or variants of diffuse large B-cell lymphomas can also be observed in cases of PCNSL: the anaplastic variant with large multinucleated tumour cells resembling Reed-Sternberg cells, T-cell rich B-cell lymphoma and intravascular B-cell lymphoma with primary manifestation within the brain or the spinal cord. HIV/AIDS-associated PCNSLs are characterised by large plasmoblastic or small Burkitt-like cells and tumour necrosis. Primary leptomeningeal large B-cell lymphomas do occur very rarely and are diagnosed by cerebrospinal fluid cytology.