We measured levels of antibodies to Japanese encephalitis virus (JEV) in serum and in cerebrospinal fluid (CSF) specimens obtained from 32 patients with acute encephalitis by using "antibody-capture" solid-phase enzyme-linked immunoassays specific for IgM or IgG to JEV. The proportions of confirmed cases with IgM to JEV detectable in CSF were 68% (obtained on day 1), 100% (day 7), 96% (day 30), and 72% (day 180). For IgG in CSF the proportions were 47% (day 1), 89% (day 7), 100% (day 30), and 100% (day 180). Twenty-five CSF samples were obtained from control patients with other diseases with possible nervous system involvement (but none with a clinical diagnosis of viral encephalitis); none had detectable IgM to JEV. Five JEV-infected but asymptomatic siblings of patients with encephalitis were also examined; all had high levels of IgM to JEV in serum, but none had detectable IgM to JEV in CSF.