Since thee beginning of the present century retroviruses have been reported as causative factors that produced transmissible tumors in birds. However, in 1980 Poiesz et al, established, for the first time, a link between a retrovirus and leukemia called Human Lymphotropic Virus Type 1 (HTLV1). Over the course of past 27 years the epidemiology of human T lymphotropic Virus type l has matured. The geographic distribution of the virus has been defined, although some puzzles persist such as the high prevalence in southwestern Japan but low prevalence in neighboring regions of Korea, China and eastern Russia and also a region of high prevalence in Iran. This virus causes two distinct diseases Adult T-cell leukemia / lymphoma and HTLV1 associated myelopathy/ tropical spastic paraparesia. The virus has worldwide spread now, although in some areas it is endemic. Transmission modes of the virus are through sexual contact, breast feeding, injecting infected products, and using infected needles. For the first time in Iran, Tabei et al, reported two Khorasani cases of adult T-cell leukemia/lymphoma with hypercalcemia in 1986 and then Farid and Tabei reported 13 other cases from Khorasan. Now it is confirmed that Mashhad is an endemic area for HTLV1 in the world. This review will discuss the epidemiology, biology, mode of transmission, diseases induced by HTLVI prevention of the disease and treatment of the patient.