The Nagasaki Prefecture, Japan (population: 1.5 million), is one of the hot endemic foci of Human T-lymphotropic virus type 1 (HTLV-1). Prevalence of HTLV-1 carriers are approximately 10% in the age group over 40 years old (40,000 individuals), approximately 10 times of the national average. Annual registry of adult T-cell leukemia (ATL) in the Prefecture is approximately 60 cases (estimated incidence: 100 cases), or a half percent of total deaths. A effective measure to control the endemic cycle of HTLV-1 has been imperative, since practical ways to prevent or control ATL are not available. A prefecture wide intervention at Nagasaki by refrain from breast-feeding blocked approximately 80% of mother-to-child transmission of HTLV-1.