Infection of HTLV-1 during childhood may be the most probable cause of leukemogenesis of ATL. The possibility of mother to child transmission of HTLV-1 was studied. Our epidemiological investigation disclosed that almost all mothers of HTLV-1 carrier children were positive for anti-HTLV-1 antibody and children born from carrier mothers showed statistically higher positive rate for anti-HTLV-1 antibody than control groups. A large number of HTLV-1 positive lymphocytes were detected in the milk from carrier mother, but not in the cord blood from newborn babies delivered from carrier mothers. We inoculated the fresh milk collected from carrier mothers into the oral cavity of a common marmoset to prove the oral infection. The marmoset was found to be seroconverted and viral antigen expression was detected in short term cultures of its peripheral T lymphocytes. These results suggest that we can prevent the transmission of HTLV-1 by prohibiting the breast-fed [corrected] by carrier mother. We have so far followed up 55 children born from carrier mothers but fed with compound milk only. None of the children in this group became a carrier of HTLV-1, whereas breast-fed group was found to have higher incidence of sero-positivity for HTLV-1. Therefore the trial prevention of HTLV-1 transmission is now undertaken in Nagasaki district.