As patients with human immunodeficiency virus infection live longer because of better antiretroviral therapy and infection prophylaxis, the incidence of non-Hodgkin's lymphoma has increased. The risk increases inversely with CD4 count--the most widely used surrogate marker for progressive immune suppression. Zidovudine itself does not appear to be a risk factor. Patients frequently present with extranodal advanced disease. The central nervous system is the primary site in 10% to 20% of cases. Important prognostic factors are performance status, a prior history of acquired immunodeficiency syndrome, and bone marrow involvement. Therapy is complicated by underlying immunosuppression, opportunistic infection, and poor bone marrow reserve. Progress has been made using colony-stimulating factors and less intensive chemotherapy regimens in systemic non-Hodgkin's lymphoma. Treatment of primary central nervous system lymphoma with radiation therapy has not improved survival.