In Japan, dengue epidemics were recorded once before the end of World War II (1942-1945) on the Main Islands and several times (1893-1955) on the Okinawa Islands. Blood samples were obtained from residents in Osaka and Okinawa, and their antibodies were examined by neutralization tests against dengue and Japanese encephalitis viruses. Of 60 serum samples each from Osaka and Okinawa, 11 and 15 sera, respectively, showed positive titers against one or more dengue serotypes. These results confirm that the epidemics had been very large. Moreover, the results showed that the epidemics had been due to dengue type 1 virus in Osaka and to dengue type 1 and 2 viruses in Okinawa.