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We report two children with acute loss of neurological functions and signs of an increased intracranial pressure. Imaging techniques ruled out space occupying lesions, whereas CSF cytology indicated CNS involvement of a non-Hodgkin lymphoma in the form of abnormal lymphocytic pleocytosis with malignancy criteria fulfilling lymphoid cells. CSF protein electrophoresis and Borrelia burgdorferi serology revealed neuroborreliosis which was successfully treated with antibiotic therapy. The malignancy mimicking cytology is based on a blastoid transformation of B- and T-lymphocytes due to the antigenic stimulus of B. burgdorferi infection. Lymphoid cells in the CSF of a patient with acute or chronic neurological symptoms raise the differential diagnosis of inflammatory etiology versus CNS lymphoma. Monomorphism and higher quantity of the lymphoid cells point to CNS lymphoma. A lower quantity and polyclonal pattern of lymphoid cells associated with an elevated protein fraction caused by intrathecal immunoglobulin synthesis suggest an inflammatory etiology.