A high human immunodeficiency virus (HIV) load may increase the probability of HIV transmission by sexual contact, but the association of virus load of hepatitis C virus (HCV) with risk of HCV transmission is uncertain. HIV and HCV virus loads were examined in hemophilic men, as were risks of HIV and HCV transmission to their female partners in a hemophilia cohort in which most subjects are dually infected. A higher HIV load was associated with an increased risk of HIV transmission (odds ratio [OR], 1.31 per log10 increase in virus load). A higher HCV load was associated, although not significantly, with an increased risk of HCV transmission (OR, 1. 42 per log10). HCV load was higher among dually infected men than in those infected with HCV alone (P=.001). However, much larger studies are needed to clearly show whether HIV/HCV coinfection significantly increases the risk of HCV transmission to female partners.