Primary central nervous system (CNS) lymphomas were studied in fifteen autopsied patients with the acquired immunodeficiency syndrome (AIDS). Using the working formulation for non-Hodgkin's lymphomas, the tumors were classified as large cell (7 patients), mixed large and small cell (6 patients), small cleaved cell (1 patient), and unclassifiable (1 patient). The mixed lymphomas displayed unusual features characterized by a high mitotic rate and the presence of numerous medium-sized cells (5 to 10 mus), not classifiable using the working formulation. Focal T cell and lymphoplasmacytoid B cell infiltrates accompanied lymphoma cells at the periphery of and remote from solid tumor masses in 9 cases. Immunohistochemical analysis of the lymphomas suggested B cell neoplasms. All of these patients had concurrent CNS and systemic cytomegalovirus (CMV) infections. The CNS infections were of both viral (CMV, human immunodeficiency virus (HIV), varicella zoster virus (VZV), progressive multifocal leukoencephalopathy (PML) and non-viral (toxoplasmosis, candidiasis) etiology. In the general AIDS population at our institution, the autopsy incidence of CNS infections and systemic CMV was 63% and 60%, respectively. In contrast, the incidence for both these entities was 0% in otherwise healthy, non-AIDS patients with CNS lymphoma supports the hypothesis that viral infection plays a role in the pathogenesis of CNS lymphoma in the immunocompromised. Polyclonal lymphoplasmacytoid B and T cell infiltrates accompanying lymphoma may produce diagnostic difficulties on surgical biopsy. As these infiltrates were a frequent feature in this study, we caution that their recognition does not argue against the presence of CNS lymphoma.