Hemorrhagic fever with renal syndrome (HFRS), scrub typhus, murine typhus and leptospirosis have been the principal acute febrile diseases in Korea for many years. To evaluate the seroepidemiologic patterns of the acute febrile illness, sera collected from 4,503 patients in 1997~1998 were examined for antibodies against Hantaan virus, Orientia tsutsugamushi and Rickettsia typhi by indirect immunofluorescent antibody technique (IFA) and macroscopic agglutination test for Leptospira interogans. Seropositive cases for Orientia tsutsugamushi, Rickettsia typhi, Leptospira interogans and Hantaan virus were 261 (12.4%), 242 (11.5%), 11 (0.5%), and 250 (11.9%) in 1997, and 415 (17.3%), 273 (11.4%), 16 (0.7%), and 357 (14.9%) in 1998, respectively. Male was affected more frequently by HFRS and leptospirosis while scrub typhus was more prevalent in female. Old age group was more susceptible to the acute febrile diseases. Most positive cases were occurred during October and November for scrub typhus, and during November and December for HFRS. These results showed similar patterns with previous epidemiological data obtained during recent several years, except the single scrub typhus epidemic in 1998, and implied that no significant changes occurred in ecologic system for acute febrile diseases in Korea.