Hemorrhagic fever with renal syndrome (HFRS), scrub typhus, murine typhus and leptospirosis have been the principal acute febrile diseases in Korea. To evaluate the seroepidemiologic patterns of acute febrile illness, sera collected from 2,423 patients in 1996 were examined for antibodies against Hantaan virus, Orientia tsutsugamushi, Rickettsia typhi, and Borrelia burgdorferi by indirect immunofluorescent antibody technique (IFA) and macroscopic agglutination test for Leptospira interogans. Seropositive cases against Otsutsugamushi, Rickettsia typhi, Leptospira interogans and Hantaan virus were 192 (7.9%), 193 (8.0%), 12 (0.5%) and 324 (13.4%), respectively. Male was more affected in HFRS and murine typhus contrasting to scrub typhus and leptospirosis in female. Most positive cases occurred during October and November for scrub typhus, and during November and December for HFRS. These results showed similar patterns with previous epidemical data for recent couple of years, and possibly implied no significant changes occurred in ecologic situations for acute febrile diseases in Korea.