Human T-cell lymphotropic viruses type 1 and 2 are retroviruses that share the same routes of transmission as HIV-1. Since these agents are prevalent simultaneously in different parts of the world, coinfection is a frequently reported event. However, prevalence rates of coinfection differ for distinct populations and regions of the world or for each virus, with human T-cell lymphotropic virus type 1 being more prevalent among HIV-1-infected individuals in the Southern hemisphere, while type 2 is more frequently found in the Northern hemisphere. In common, they share the tropism for T-lymphocytes, although human T-cell lymphotropic virus type 1 and HIV-1 are predominantly CD4+ T-cell tropic and human T-cell lymphotropic virus type 2 preferentially infects CD8+ cells. The biological properties of HIV-1 are distinct of those found in human T-cell lymphotropic virus 1/2. This fact makes possible an in vivo interaction between these agents, when coinfecting the same patients, with potentially relevant clinical implications. The available evidence suggests a protective role for coinfection by human T-cell lymphotropic virus type 2 on AIDS progression. This hypothesis is supported by several laboratory evidences, as well as by a number of clinical studies that found no significant interaction between human T-cell lymphotropic virus type 2 and HIV-1, or even detected a protective effect on HIV-1 disease. On the other hand, human T-cell lymphotropic virus type 1 seems to be a significant cofactor, with a potentially important role in HIV-1 infection. Although the clinical evidence is still controversial with regard to the real impact that coinfection exerts on clinical evolution, the majority of studies suggest it is associated with a modification of the natural history of HIV-1 infection, with a faster clinical progression and a shorter survival time. The main limitation of the available data is due to methodological problems in the majority of studies, which weaken the validity of their conclusions. A common finding in coinfection by both human T-cell lymphotropic virus type 1 and 2 is the increase in CD4+ cell count, but without any additional immune benefit for patients. Due to the limited available data, we need more, larger studies, designed to respond to the pending questions on the real significance of coinfection by these retroviruses.