Human T-lymphotropic virus type I (HTLV-I) and HTLV-II have been postulated to accelerate disease progression in patients coinfected with HIV. However, recent evidence suggests that HTLV-II has no effect on HIV disease progression. In addition, it has recently been reported that HIV viral load was not increased in subjects coinfected with HTLV-I, suggesting that the biologic basis for the hypothesis does not exist. Several recent studies in Brazil, however, suggest that coinfection with HTLV-I and HIV has substantial medical consequences. For example, coinfection was associated with a higher CD4 lymphocytes count following adjustment for HIV viral load or HIV clinical stage. In addition, coinfected subjects have a high prevalence of clinical significant myelopathy. The effect of other putative viruses as cofactors in HIV disease progression is also discussed.