Okinawa prefecture is one of the endemic areas for adult T-cell leukemia/lymphoma (ATLL) in Japan. In this study, 2,013 serum specimens drawn serially over a period of 15 years (1968-1983) from 311 mother/child pairs in Okinawa were tested for antibodies to human T-cell leukemia virus type I (HTLV-I) by enzyme-linked immunosorbent assay and by indirect immunofluorescence. The prevalence rate of HTLV-I antibodies was 20.9% (65 cases) in the mothers and 3.2% (10 cases) in the children. Of the 65 seropositive mothers, 10 (15.4%) had seropositive children. This study revealed a significant difference between the prevalence rates of HTLV-I antibodies in mothers and children. In addition, children born to seropositive mothers had acquired their HTLV-I antibodies by the age of 3 years, and were still seropositive at the age of 18 years. No initially seronegative child was found to have seroconverted during the period investigated.