In exceptional cases, patients with aseptic meningitis eventually develop aseptic meningoencephalitis. To find a candidate marker for the development of aseptic meningoencephalitis in adult patients diagnosed with aseptic meningitis, we compared 12 different cytokines/chemokines in cerebrospinal fluid (CSF) from 5 patients with aseptic meningoencephalitis, 8 patients with aseptic meningitis, and 8 patients with control disease. Only the CXCL13 concentration was significantly elevated in the CSF of the group with aseptic meningoencephalitis compared with the group with aseptic meningitis. Thus, CSF CXCL13 may be a useful marker for predicting the prognosis of aseptic meningitis.