Central nervous system lymphomas are rare aggressive tumors requiring a prompt diagnosis for successful treatment. The standard procedure for diagnosis is stereotactic biopsy, but it typically takes over 2 days for histopathology results. For a few years, a new strategy for detecting lymphoma cells has been published, which involves analyzing brain biopsy rinse fluid (BBRF) using multiparameter flow cytometry (MFC) and cytology. This approach, now used in some specialized centers, warrants the diagnosis of B-cell lymphoma in less than 24 h and, when combined with histopathology, allows for a multidisciplinary diagnosis. This chapter describes the processing of the sample, highlighting the time of processing and transport, the analysis by cytology and MFC, and the interpretation, including potential pitfalls such as corticosteroid treatment and blood contamination. Finally, the interest of storing lymphoma cells for research purposes in this rare disease is discussed.