Twenty years after the discovery of the human T-cell lymphotropic virus type I, quite great progress has been achieved and many epidemiological, clinical and phylogenetic features are well known. Seroprevalence increases with age in endemic regions and is higher in women. Three diseases are clearly associated to the virus: tropical spastic paraparesis/myelophathy, adult T cell leukemia and uveitis. The transmission is similar to other retroviruses: contact with contaminated blood, sexual intercourse, and mother to child transmission, mainly through breastfeeding. The natural history of diseases associated to the human lymphotropic virus has not been well established yet. Further studies are required to estimate correctly the risk of infected carriers developing diseases in their life-time. In the last 5 years, reports have pointed towards the association of a syndrome like tropical spastic paraparesis and the human lymphotropic virus type II. The uncertain prognosis for people infected with both types of viruses and the transmission through sexual contact, blood transfusion and from mother to child make this infection a public health problem, mainly in endemic regions.