Purpose. Diagnosis of WNV (WNV) relies upon serologic testing which may take several days after the onset of clinical symptoms to turn positive. Anecdotal reports suggest the presence of plasma cells or plasmacytoid lymphocytes in the cerebrospinal fluid (CSF) may be an early indicator of WNV infection.Methods. The CSFs of 89 patients (12 with WNV, 12 with other viral illness{OVI}, and 65 with nonviral illness{NVI}) were compared for the presence of either plasma cells or plasmacytoid lymphocytes.Results. Plasma cells were rarely seen in any of the patients. Plasmacytoid lymphocytes were more commonly seen in WNV (58%) and OVI (50%) than NVI (11%). The differences were significant for WNV versus NVI, but not WNV versus OVI (P<0.001andP=0.58,resp.).Conclusions. A CSF pleocytosis with plasma cells or plasmacytoid lymphocytes was neither sensitive nor specific for the diagnosis of WNV infection.