Sera of patients visited at the Kangnam St. Mary Hospital in Seoul were collected randomly at the Department of Clinicopathology from January, 1998 to December, 2002. Specimens were collected two twice a month, in a 15-day interval, and 100 specimens were collected at a time. Specimens test in duplicate, and/or displaying antinuclear antibody reaction were excluded from the seroepidemiological analyses. Detection of antibodies to Hantaan virus, an etiologic agent of hemorrhagic fever with renal syndrome (HFRS), was done by indirect immunofluorescent antibody (IFA) technique. Out of 11,361 sera tested, 445 cases (3.9%) showed specific antibody to Hantaan virus. Sexual difference was not noted. Annual incidence of HFRS cases showed a 3 year-periodicity. In the monthly incidence analysis, two peaks of incidence were appeared in the male cases, the first peak in March and the second in August. Female cases showed a single peak in October. The age distribution showed that 64.9% of the sero-positive cases were from 40 to 69 years of age. Peak age-group was in the 6th decade. Each decade of age-group showed diverse patterns of annual and monthly incidences. These results suggest the incidence of HFRS shows a periodicity and a unique pattern in each age group.