Suspected dengue cases in Japan during the period of 1985-1995, 173 cases with unknown febrile illness entering or returning from mostly southeast Asia, were tested by serology and RT-PCR gene amplification. Seventy seven cases were confirmed by a significant rise of dengue 2 (Den 2) HI antibody in paired sera or by detection of HI antibody titer higher than 1:320 in single serum. The other 3 cases with antibody levels not higher than 1:80 in paired sera and 12 cases with an antibody 1:160 in single sera were positively suspected of dengue infection but were not confirmed. Countries of origin of confirmed cases were Thailand (39 cases), Philippinse (15), India (13), and Indonesia (9). Sera of dengue cases showed high degrees of cross reactivity of Japanese encephalitis virus (JEV) in HI test but not in IgM capture ELISA. Sera of confirmed JEV-infected cases, however, showed practically no cross reactivity to Den 1 4 in HI test, suggesting unilateral cross reactivity of HI antibody. RT-PCR detected the Den 1 genome in sera of 3 cases obtained within 3 days after onset and the Den 2 genome in serum of case 4 days after onset. Although the number is limited, 92 (53%) out of 173 cases of febrile illness of unknown etiology entering Japan from tropical countries were either confirmed or positively suspected to be dengue fever. Considering possibilities of under reporting, importations of tropical viral infections should be bigger in number and will necessitate our intensified alertness.