The cytomorphologic findings in the cerebrospinal fluid (CSF) of four patients with lymphocytic meningoradiculitis (
Lyme disease) related to a tick bite (due in at least two cases to Borrelia burgdorferi) are reported. In all cases, the May-Grünwald-Giemsa-stained centrifuge preparations of the CSF showed a cellular pattern consisting of a lymphocytic pleocytosis composed mainly of immunoblasts and plasma cells associated with numerous foamy macrophages. Direct immunofluorescence studies in one case showed the polyclonal pattern of the immunoblasts and the plasma cells. These CSF findings can be considered as suggestive of the spirochetal origin of a lymphocytic meningitis following a tick bite.