Abstract Plasmablastic lymphoma (PBL) is a rare subtype of diffuse large B cell lymphoma and commonly presents as an oral mass in HIV patients. Extraoral PBL has been reported, including one case of primary central nervous system PBL (PCNSPBL). The cytological features of PBL have been described, including cerebrospinal fluid (CSF) cytology findings for secondary CNS involvement by PBL. The etiology of PCNSPBL is still unknown. We report here the CSF cytology of a PCNSPBL, which shows a hypercellular specimen composed of markedly atypical, singly dispersed plasmacytoid cells with frequent abnormal mitoses and binucleation. The neoplastic cells are positive for CD138. Flow cytometry of the CSF specimen demonstrates a monoclonal neoplastic cell population, which is CD138 positive, kappa light chain positive, lambda light chain negative, and CD19 negative. Molecular analysis and immunohistochemical stains on a tissue biopsy confirmed the diagnosis and reveal concurrent infections with Epstein‐Barr virus and human polyomavirus JC virus. Clinical and radiological correlations are reported, and the literature is reviewed. To the best of our knowledge, this is the first case report for CSF cytology of a PCNSPBL, demonstrating the utility of the cytological examination in the triage and diagnosis of this disease. Because of its dismal prognosis, it is critical for cytopathologists to be aware of the entity and recognize the neoplastic cells in CSF specimen. This report also emphasizes the importance of clinical and radiological correlation in the diagnosis of this lethal disease. Diagn. Cytopathol. 2011. © 2010 Wiley‐Liss, Inc.