To clarify events related to age dependence of human T-cell lymphotropic virus type I (HTLV-I) seroprevalence, the authors carried out two seroepidemiologic studies at a 9-year interval on residents of Yonaguni Island, Okinawa, Japan. The overall prevalence was 17.8% (345/1,943) in 1980 and 17.1% (264/1,543) in 1989-1990. The authors studied the relation of the HTLV-I infection rate to the birth year of 967 individuals tested in both years. Of the 967 individuals, 209 were positive for anti-HTLV-I in the 1980 survey. By the 1989-1990 survey, only 7 of the 758 anti-HTLV-I negative cases (0.9%) had become positive for anti-HTLV-I, and all 7 were over 40 years of age. For persons born in the same period, there was almost no difference between the prevalence of HTLV-I found in the 1980 survey and that found in the 1989-1990 survey. These data show that there were few new cases of HTLV-I infection and none in the lower age groups over the 9 years, which suggests that higher rates of HTLV-I infection in adults reflect past higher rates of infection as infants.