Primary central nervous system non-Hodgkin's lymphomas are observed in approximately 1.9% of all patients with acquired immunodeficiency syndrome (AIDS). The yearly incidence of AIDS-associated tumors has surpassed the yearly incidence from all other causes and could become as frequent as low-grade astrocytomas by 1991. Patients' signs, symptoms, and radiographic studies are not specific for this lesion; brain biopsy usually is necessary to make a definitive diagnosis. Most tumors are high-grade lymphomas and are pathologically similar to the primary central nervous system lymphomas observed before the AIDS epidemic. AIDS-associated tumors respond readily to radiation therapy. However, patient survival remains limited owing to other manifestations of the syndrome.