To establish a desirable preventive measure against mother-to-child transmission of HTLV-I through breast milk, we conducted a prospective study to investigate the seroconversion rate among children born to HTLV-I carrier mothers on two highly HTLV-I-endemic islands where 8% of pregnant women carry HTLV-I. Between 1985 to 1991, 428 pregnant women were found to be positive against anti-HTLV-I antibody and were advised not to breast-feed their newborn babies. Among them, 212 women (50%) accepted this advice and the other mothers proceeded to breast-feed. Results were obtained from 277 children born to HTLV-I carrier mothers and were followed up until more than 30 months of age. When the seroconversion rate was analyzed by feeding manner, short-term breast-feeders (< or = 6 months) showed a statistically significant lower seroconversion rate than long-term breast-feeders (2/51; 3.9% vs. 13/64; 20.3%, p < 0.05). On the other hand, four out of 162 bottle-fed children (2.5%) became positive. It is hypothesized that maternal HTLV-I antibody may protect babies from HTLV-I infection through breast milk during the first 6 months.