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Human T-cell lymphotropic virus type I (HTLV-I), the cause of adult T-cell leukemia/lym phom a (ATLL) and HTLV-I associated myelopathy/tropical spastic paraparesis (HAM/TSP), is widespread in the Pacific basin. Modes of virus transmission include blood transfusion (and intravenous drug use), breast milk, and sexual intercourse. High prevalences of HTLV-I infection and disease occur among the inhabitants of southwestern Japan and among first and second-generation (issei and nisei) Japanese-Americans in the Hawaiian Islands. Other Pacific populations with high prevalences of HTLV-I infection include several remote groups in West New Guinea, Papua New Guinea, the Solomon Islands, and Vanuatu, which have had no contact with Japanese or Africans. By contrast, Micronesian and Polynesian populations, even those with prolonged contact with Japanese, exhibit low prevalences or no evidence of HTLV-I infection. Low prevalences of infection are also found in Australia, except among some aboriginal populations. Changing patterns of HTLV-I infection and disease are no better exemplified than in Japan, where striking reductions in transfusion-acquired infection and subsequent development of HAM/TSP have followed the institution of nationwide screening of blood donors for HTLVI infection. Furthermore, virus transmission from mother to infant by means of infected breast milk has been markedly curtailed in HTLV-I-hyperendemic regions in Japan by interrupting the practice of breast feeding by HTLV-I-infected mothers. The next frontier of HTLV-I research is in Melanesia, where highly divergent sequence variants of HTLV-I have been discovered.