Human T-cell lymphotropic virus type I (HTLV-I) belongs to the family Retroviridae and subfamily Oncovirinae. HTLV-I has an outer envelope consisting of a lipid bilayer and two types of glycoproteins – gp21 and gp46. HTLV-I viral genome is a positive single stranded molecule of RNA. It containes structural (gag, pol and env) and regulatory (tax, rex) genes. Special segments, called LTR (long terminal repeat), are located at both ends of proviral DNA. The most important routes of transmission of HTLV-I are sexual intercouse, transfusion of infected blood and mother-to-child transmission via placenta and breastfeeding. There are between 10 and 20 million HTLV-I infected people in the world. HTLV-I is endemic in Japan and some Asian countries (China, Korea, Taiwan), Caribbean islands and central Africa. High risk groups (homosexuals, intravenous drug users, prostitutes) are the main source of infection in Europe and the USA. HTLV-I infected individuals are mostly asymptomatic virus carriers, and only a small proportion of them can develop a special form of T-cell leukemia or myelopathy.