Co-infections of hepatitis C virus (HCV) and either human immunodeficiency virus type1 (HIV-1), human T-cell lymphotropic virus type 1 (HTLV-1) or type 2 (HTLV-2) have beendescribed as having an impact on HCV viremia and subsequent disease progression. HCVload in serum samples from 622 patients (343 males, 279 females; median age 50.8 years)from São Paulo/southeast Brazil was analyzed using the Abbott Real Time HCV assay(Abbott Molecular Inc., IL, USA). Samples were obtained from HCV-monoinfected (n = 548),HCV/HIV-1- (n = 41), HCV/HTLV-1- (n = 16), HCV/HTLV-2- (n = 8), HCV/HIV/HTLV-1- (n = 4), andHCV/HIV/HTLV-2-co-infected (n = 5) patients, and results were compared among the groupsand according to sex. The median HCV load in HCV-monoinfected patients was 5.23 log10IU/mL and 0.31 log10higher in men than in women. Increases in viral load of 0.51 log10, 0.54log10, and 1.43 log10IU/mL were detected in HCV/HIV-1-, HCV/HTLV-1- and HCV/HIV/HTLV-1-co-infected individuals, respectively, compared with HCV-monoinfected counterparts. Incontrast, compared to HCV/HIV co-infected patients, HCV/HTLV-2-co-infected patients hadan HCV load of 5.0 log10IU/mL, whereas HCV/HIV/HTLV-2-co-infected patients had a medianload 0.37 log10IU/mL lower. Significant differences in HCV loads were detected, with malesand HCV/HIV-1- and HCV/HIV/HTLV-1-co-infected patients presenting the highest values.Conversely, females and HCV/HTLV-2-co-infected patients exhibited lower HCV loads. Over-all, HCV viremia is increased in HIV and/or HTLV-1-co-infection and decreased in HTLV-2co-infection. (AU)