Aggressive B-cell non-Hodgkin's lymphoma (B-NHL), Lymphoblastic Lymphoma and Burkitt's Lymphoma are at high risk of CNS relapse. For this reason is recommended a prophylaxis therapy. This suggests that negative leptomeningeal localization patients at diagnosis are really positive and that would be a very high risk of CNS relapse subtype patients. It's very important detecting neoplastic cells in cerebrospinal fluid as soon as possible. This study will evaluate only patients that have been flow cytometry and cytology at diagnosis.