This study will evaluate the efficacy and safety of MabThera plus high dose methotrexate plus high dose cytarabine in patients with central nervous system non-Hodgkin's lymphoma. Eligible patients will receive a treatment regimen consisting of MabThera (750mg/m2 iv) plus methotrexate (8g/m2 iv) given at intervals up to week 22, plus cytarabine (2g/m2 iv) at week 11 and week 22. The anticipated time on study treatment is 3-12 months, and the target sample size is <100 individuals.