Sixty-eight primary malignant lymphomas of the CNS exclusively confined to the brain and its leptomeninges from a series of about 8000 intracranial neoplasms (incidence 0.85%) were examined and classified according to current histopathologic criteria. Average age at onset of symptoms was 55 years, mean duration of illness to time of diagnosis was 3 months. Survival averaged 1,8 months with supportive care, but 17,2 months with surgery, radiation and/or chemotherapy. CSF cytology was a useful and reliable tool for clinical diagnosis. The cerebral hemispheres were affected in about 50%, the basal ganglia in 18%, posterior fossa in 10%, while multifocal lesions amounted to 22%. All CNS tumors were of the diffuse type of non-Hodgkin’s lymphomas; no follicular (germinal center) lymphomas were observ. d. Three histological patterns comparable to extraneural lymphomas were distinguished: Immunoblastoma (reticulosarcoma) occurred most frequently (58.8%), lympho-plasmacytoid immunocytoma constituted 28 percent, while lymphoblastic lymphoma occurred least frequently (13.2%). There were no significant differences with regard to onset, location, growth pattern or clinical course except for a much poorer prognosis of lymphoblastic lymphoma. Although there are no definite cytological differences between malignant lymphomas arising in extraneural sites or as primary lesions in the CNS, the latter showed a much greater proportion of phagocyting histiocytes (and microglia) and a frequent occurrence of plasmacytes and their precursors which apparently exceeded pure host reaction. The prognostic value of modern classification schemes for CNS lymphomas needs further critical evaluation.