To determine the proportion of acute undifferentiated fevers without neurologic deficits related to infection with Japanese encephalitis (JE) virus, flavivirus serology (dengue and JE) was performed in a cohort of 156 adults presenting to a hospital in Chiangrai, Thailand. Recent flavivirus infection was diagnosed for any individual with an IgM result > 40 units. A ratio of dengue virus IgM to JE virus IgM < 0.91 defined a JE virus infection. Diagnostic criteria for Japanese encephalitis were met in 22 individuals (14%), and were unequivocal in 8 patients. The admission findings in these eight subjects were similar to those described for other flavivirus infections. Thrombocytopenia was the most striking laboratory abnormality (median platelet count = 119,000/mm3, range = 44,000-236,000/mm3). Headache (75%), nausea (50%), myalgia (38%), rash (38%), and diarrhea (25%) were the most frequently encountered signs and symptoms. Infection with Japanese encephalitis virus is an underappreciated cause of acute undifferentiated fever in Asia.