 The incidence of primary central nervous system lymphoma PCNSL has increased threefold in immunocompetent populations in recent years. Improvements in treatment, particularly involving combined modality therapy with chemotherapy and radiotherapy have been shown to augment patient survival with a median disease-free period of about 40 months. However, the combination of these two modalities often increases the risk for delayed neurotoxicity. There is a paucity of studies that have assessed neuropsychological functioning and quality of life in patients with PCNSL. The majority of studies reported performance status and survival rates, but systematic cognitive evaluations were only seldom included. Unfortunately, relying only on these variables does not adequately assess the more subtle cognitive impairments that most patients with brain tumors experience. Neuropsychological difficulties often interfere with disease free patients ability to function at premorbid levels at work and at home. A study including neuropsychological evaluations of a relatively large group of patients with PCNSL who received combined modality treatments, and are in remission from their disease is planned. A follow-up assessment also will be performed in order to monitor performance over a specified period of time. The proposed study will also test the hypotheses that: 1 the presence of polymorphisms that influence methionine metabolism places PCNSL patients treated with chemotherapy alone or in combination with radiotherapy at risk for developing treatment-induced white matter disease and cognitive dysfunction; 2 the possession of the apolipoprotein E APOE є-4 allele is associated with the development of cognitive difficulties following treatment for PCNSL. Research in order to better understand the incidence, extent, and severity of treatment-induced neuropsychological impairments in patients with PCNSL is of utmost importance, given the recent increase in both the number of cases diagnosed and longterm survival. It is likely to provide valuable information regarding specific cognitive domains that should be addressed in the development of strategies for cognitive rehabilitation or other interventions that may be appropriate. The findings of this study will also be relevant for comparison with ongoing and future research investigating the potential neurocognitive sequelae of alternative treatment modalities for PCNSL e.g., high-dose chemotherapy followed by peripheral blood progenitor cell PBPC transplant. 