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Leukemic involvement of the central nervous system (CNS) in previously undiagnosed chronic lymphocytic leukemia (CLL) is very rare. We report the case of a 62-year-old man with neuroborreliosis in which cytologic, immunocytochemical, and flow cytometry analyses revealed the presence of clonal B-lymphocytes in the cerebrospinal fluid (CSF). After the patient received antimicrobial therapy, his meningeal symptoms cleared up, and the number of cells in the CSF decreased. Monoclonal lymphocytes were still detectable at the same percentage, however, despite systemic chlorambucil therapy. The application of intrathecal dexamethasone therapy led to the disappearance of B-cell CLL (B-CLL) cells in the CSF. We presumed that the neuroborreliosis enabled the transmigration of leukocytes, including B-CLL cells, across the blood-brain barrier via activation of matrix metalloproteinase 9, an enzyme known to open the blood-brain barrier.