Since 1974, no Japanese encephalitis (JE) case had been reported on Okinawa island in either Okinawan people or US servicemen. In 1991, three US marines stationed on Okinawa island developed encephalitis symptoms. Neutralization (N) test and IgM-capture ELISA were carried out on the serial samples of serum and cerebrospinal fluid (CSF) taken from the patients. In each patient N test on both serum and CSF samples gave a significant rise in JE antibody titer in the comparison between the acute and convalescent phases, indicating that all the cases were infected with JE virus. The IgM-capture ELISA also showed a significant rise of antibody titer of the serum and CSF samples in the convalescent phase in patients 2 and 3, while in patient 1 a significant rise in IgM antibody was observed in the serum sample, but not in the CSF sample. None of the patients had been administered JE vaccine. This report underscores the importance of JE vaccination.