In order to investigate the infection rate of Hantaan virus in Taiwan, a total of 6,536 human serum samples were collected from residents, selected by stratified random sampling, from 19 townships covering four different ethnic groups: Aborigines, Fukien Taiwanese, Hakka Taiwanese, and Mainland Chinese. Serum samples were screened for Hantaan virus antibodies by indirect immunofluorescence. The prototype Hantaan virus (76/118)-infected Vero E6 cells were used as the viral antigen for the antibody detection. Among 6,536 human serum samples, 403 (6.2%) samples had Hantaan virus antibodies. The seropositive rates for males and females were 6.1% and 6.2%, respectively. A higher seropositive rate was found among Aborigines on the Orchid Islets (11.5%) and Fukien Taiwanese on the Penghu Islets (1 1.6%), while the lowest rate was observed among Hakka Taiwanese in the south of Taiwan (2.5%). In comparing with different ethnic groups, the highest prevalence was found among Fukien Taiwanese (8.1%) and the lowest among Mainland Chinese (4.9%). Among the different geographical areas, the highest positive rate was found i n western Taiwan (7.1%) and the lowest in southern Taiwan (5.4%). Hantaan virus antibodies were also detected in 22 of 548 (4.0%) rat serum samples. The highest seropositive rate was found in rat sera collected from the Orchid Islets (21.4%). None of the rat sera collected from Hsinchu, Miaoli, Changhua, Nantu, Yunlin, Chiayi, Tainan, and Penghu Counties were positive. Hantaan virus antibodies were found in rats: Rattus rattus (20%), Bandicota indica (9.0%), Rattus norvegicus (8.3%), Bandicota nemorivaga (6.3%), Rattus losea (4.2%), and Apodemus agrarius (1.6%). Hantaan virus antibodies were not detected in rat sera collected from species of Rattus coxinga, Rattus culturatus, Mus musculus, Mus carol;, Suncus murinus, and Apodemus semotus. The results show that the Hantaan or Hantaanrelated virus exists and is distributed widely in both human and rats in Taiwan.