The incidence of primary central nervous system lymphoma (PCNSL) is increasing, not only in immunodeficiency states, but also in apparently normal individuals. The most common presentation of PCNSL is that of an intracranial mass lesion. Ocular involvement is associated in 20% of patients. CT/MR scan typically shows one or several periventricular tumors with indistinct margins that diffusely and densely enhance following contrast infusion. The diagnosis relies on lumbar puncture, vitreous biopsy, or stereotactic biopsy of a brain lesion demonstrating lymphomatous cells. If possible, corticosteroids should be used only after definite diagnosis. Corticosteroids have a cytotoxic effect responsible for transient remission in 40% of patients. Whole brain radiation therapy induces a complete or partial response in 80% of patients but recurrence always occurs and the median survival does not exceed 14-18 months. The addition of systemic and intrathecal chemotherapy seems to substantially improve the prognosis with median survival exceeding 3 years in some studies. PCNSL associated with AIDS generally occurs at a late stage of the disease and is the fourth cause of death in AIDS patients. Radiation therapy is useful but the median survival does not exceed 5.5 months because patients most often die of opportunistic infections.