BACKGROUND: Human T-cell lymphotropic virus type I (HTLV-I) is a retrovirus that has been identified as a cause of adult T-cell leukemia/lymphoma and tropical spastic paraparesis. HTLV-I infection is highly endemic in the southwestern islands of Japan, Caribbean basin, South America, and Africa. In 1993, we showed that the seroprevalence of antibodies to HTLV-I was 0.13% among blood donors in Korea, but surprisingly, 0.80% in Cheju-Do adjacent to endemic areas of Japan. So this study was designed to reevaluate the seroprevalence of antibodies to HTLV-I among residents in Cheju-Do. METHODS: Total 2,372 residents in Cheju-Do were tested from December 1995 to March 1996. Anti-HTLV-I antibodies were detected by the microtiter particle agglutination test. RESULTS: Among total 2,372 residents, 19 were anti-HTLV-I positive. So the overall positive rate of anti-HTLV-I antibodies was 0.80%. The positive rate in females was higher than in males (0.82% vs 0.78%). The positive rate was 1.45% in the age group of 20-29 years, 1.41% in 40-49 years, 0.91% in 0-9 years, 0.70% in 30-39 years, and 0.54% in 50-59 years. The mean age of seropositive cases is 35.2 in males and 35.4 in females, with a mean of 35.3. Geographically, high positive rate was observed in Sogwipo-City (1.37%) and Namcheju-Gun (0.83%) compared to those of Pukcheju-Gun (0.64%) and Cheju-City (0.61%), which showed high seroprevalence in districts adjacent to endemic areas of Japan. Any specific risk factors or associated disorders of HTLV-I infection could not be found among the seropositive cases. CONCLUSION: The seroprevalence of antibodies to HTLV-I in Cheju-Do was noted to be very high by the microtiter particle agglutination test. So henceforth serosurvey by confirmative laboratory tests is needed, and if high seroprevalence is showed from it, screening of blood donors for HTLV-I in Cheju-Do should be considered to prevent transfusion-associated HTLV-I infection.