Hantaan virus is the etiological agent of korean haemorrhagic fever, a serious form of haemorrhagic fever with renal syndrome (HFRS). Serodiagnosis of HFRS among fever of unknown origin patients was made by indirect immunofluorescent antibody (IFA). Seroepidemiological analyses including sex, age and seasonal or regional distribution of HFRS patients in Korea from 1994 to 2000 were performed. 1. Of 2,204 sera from acute febrile disease patients tested from 1994 to 2000, 234 cases of HFRS showed serological response and among the 234 seropositives, the ratio of males to females was 3.5:1. 2. Seasonal incidence of HFRS cases shows that more than 70% of cases occurred from October to January and reached maximum in November and began to decrease gradually from February on. 3. About 67.5% of total HFRS cases between 1994 and 2000 were those living in Gyeonggi, Seoul and Chungbuk province. 4. The age distribution revealed that 88% of the seropositives were from 21 to 70 years of age, peak decade was 5th decade. In summary, 234 seropositive HFRS cases revealed that males to females were 3.5:1, 5th decade as the peak age, November as the highest season, Gyeonggi, Seoul and Chungbuk province were the most prevalent residence area.