We examined the possible relationships between hepatitis C virus (HCV) viral load and host factors, viral factors, and anti-HIV therapy in 379 HIV/HCV-co-infected patients. Multiple linear regression analysis identified two independent factors associated with higher HCV viral load, HCV genotype 1 or 4 infection and protease inhibitor-based antiretroviral therapy. Antiretroviral therapy in general was independently associated with lower HCV viral load. This suggests that HCV viral load kinetics could differ according to the choice of HAART regimen.