Human T lymphotropic virus-1 (HTLV-1) is a retrovirus which infects T lymphocytes (CD4+) to cause adult T cell leukaemia/lymphoma (ATL), tropical spastic para-paresis and several other HTLV-1 associated disorders. ATL has been reported worldwide but areas of high incidence include Japan (particularly the south-west), Central and South America, northern Iran, West and Central Africa and Melanesia. In the general Japanese population, HTLV-1 carriage is 0.1% but this can be as high as 50% in endemic areas. Six per 10000 carriers are estimated to progress to ATL each year. The three major routes of infection are mother to baby through breast-feeding, sexual intercourse and blood transfusion. There is a lengthy latency period of up to 40 years before the development of ATL. Up to 50% of ATL patients present with a cutaneous eruption. Diagnosis is established by the detection in lymphocytes of monoclonal integration of HTLV-1 proviral DNA. Even with aggressive treatment, ATL patients generally have a poor prognosis.