Transmission of HTLV-I via mother's milk has been confirmed by epidemiological studies and by animal experiments using carrier mother's milk and a marmoset. The prevalence rates of HTLV-I carriers in children born of carrier mothers in endemic areas and ATL families were higher than in three control groups of young people. Also, the prevalence rates of carrier mothers who were traced from previously identified carrier children in three groups (two endemic areas and ATL family members) were extremely high. When HTLV-I antigen-positive lymphocytes were detected in carrier mother's milk, the child infection rate was higher than in the cases where antigen-positive cells could not be detected in mother's milk. The number of infected cells present in carrier mother's milk was calculated and the volume of milk given to the baby from delivery to weaning was estimated. Then, an equivalent amount of carrier mother's milk was inoculated into a marmoset orally and this marmoset seroconverted 2.5 months after the inoculation. A campaign to stop carrier mothers from giving their breast milk to their babies has been started in Nagasaki. So far, this trial has been shown to be successful in the prevention of mother-to-child infection.