BACKGROUND: Spotted fever group rickettsiosis occurs worldwide and includes various causative organisms depending on the region and clinical features. In Korea, previous studies have shown that several kinds of spotted fever rickettsiae have been identified in ticks, and in stored sera obtained from febrile patients. Previously, it was difficult to correlate the results of serologic or molecular biologic tests with the clinical or epidemiological features of this disease in humans. In 2004, the first Korean patient with Japanese spotted fever (JSF) was identified on Mueui Island, Incheon, Korea. To estimate the prevalence of JSF and to compare the incidence of JSF with those of other infectious diseases endemic to Korea, we performed a serosurvey of Japanese spotted fever and other rickettsiosis (scrub typhus and murine typhus), hemorrhagic fever with renal syndrome and leptospirosis on the island where the patient had been living. MATERIALS AND METHODS: In October 2004, we performed a seroprevalence survey of Mueui Island where nearly 300 persons resided. There were 91 persons who participated in the survey and answered the questionnaire. The participants included 30 healthy subjects receiving a check up at the Health Promotion Center at Inha University Hospital, and 30 patients with rheumatoid factor as control groups for the serologic tests. RESULTS: Of the 91 residents, only one person showed a positive reaction to R. japonica at a titer of 1:80. IgG antibodies against O. tsutsugamushi were positive at a titer of 1:32 in 3 persons, and those against R. typhi were at 1:32 in 1 person and at 1:64 in 2 persons. Serum IgG antibodies to Hantan virus were positive at a dilution of 1:64 in 2 persons and those to leptospira were negative. All 30 healthy persons and 30 patients with rheumatoid factor in the control group showed negative results in 1:40 diluted sera. CONCLUSION: This study demonstrates that the seroprevalence of R. japonica is not as high in Korea as it is in Japan. Further studies should be performed in a large number of patients, including residents of other islands and the Korean peninsula.