From 1983 to 1986, 1,813 children in nursery schools in Ishigaki Island and 1,228 children under 15 years old in the rural area in the Yaeyama District of Okinawa, Japan. were tested for anti-HTLV-I, 18 children (1.0%) in Ishigaki Island and 39 children (3.2%) in the rural area were positive. In order to survey when anti-HTLV-I developed in these children, their older serum samples were investigated retrospectively for 1 to 5 years. Two cases of seroconversion from anti-HTLV-I negative to positive were found between the age of 2 and 4, and there were no cases of seroconversion over 4 years old. Among the children who suffered maternal transmission of HTLV-I and developed an ti-HTLV-I before the age of 15, about 80",) of the children were considered to have anti-HTLV-I before the age of 2, and the remainder developed anti-HTLV-I before the age of 4.