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Abstract 5094 Background. Leptomeningeal involvement is an adverse complication of haematological malignancies, acute leukemias and lymphomas. The diagnostic gold standard to identify cerebrospinal fluid (CSF) involvement is conventional cytological analysis, technique characterized by low sensitivity and specificity. Recent papers make evidence that the efficiency of CSF involvement detection could improve by use of Flow Cytometry (FCM) immunophenotypic analysis of CSF samples. We describe the FCM method used in our hospital and we compare the results with cytological findings. Methods. CSF samples should be processed immediately, within 1 hour after collection to avoid cell's deterioration; in B Non Hodglin Lymphomas (B-NHL) we use two steps FCM approach, in four color staining (FITC/PE/PerCP/APC). The first step was two tubes: kappa/lambda/CD45/CD19 and CD5/CD20/CD45/CD3 or CD3/CD20/CD45/CD10 on the basis of a previous know immunophenotype or not. If a pathological population is identified, we complete immunophenotype characterization with a second step, using antibody panels suitable for the suspected malignancy. In case of suspected meningeal relapse of Acute Leukemia we use a monoclonal antibody combination select for Minimal Residual Disease. Results. Between 2006 and 2009 we studied by FCM analysis 53 CSF samples of 36 patients. 7 CSF of 29 B-NHL newly diagnosed resulted positive at FCM analysis (24,1%), 1 CSF of 7 Acute Leukemias shows disease relapse. The conventional cytology was positive only in 1 case of B-NHL. Conclusion. Our study confirms that immunophenotypic flow cytometric analysis of CSF cells markedly improves sensitivity to identify CSF involvement in haematological malignancies. Disclosures: No relevant conflicts of interest to declare.