Primary central nervous system lymphoma, especially that associated with AIDS, is increasing in frequency. CT scan is virtually diagnostic when it shows multiple, hyperdense, diffusely enhancing periventricular lesions. Treatment often results in a complete remission, but the overall prognosis is poor. Surgery is important diagnostically, but resection is not helpful. Corticosteroids can produce significant shrinkage, and even disappearance, of PCNSL, but cranial radiation is still the cornerstone of therapy. The addition of chemotherapy to RT is promising in non-AIDS patients but still investigational.