The effect of highly active antiretroviral therapy (HAART) on liver function and viral load of hepatitis C virus (HCV) was studied in 21 hemophilic men coinfected with HCV and human immunodeficiency virus (HIV). HCV RNA polymerase chain reaction was measured by branched DNA Quantiplex assay on frozen plasma samples obtained at baseline and at 24, 48, and 96 weeks after initiation of HAART. HCV RNA increased at 48 and 96 weeks after initiation of HAART therapy (198x105 Eq/mL [P=.03] and 227x105 Eq/mL [P<.0001], respectively, compared with baseline [141x105 Eq/mL]). This increase was associated with an increase in CD4 cell count and reduction in HIV viral load but no change in hepatic transaminases. With discontinuation of HAART, HCV RNA decreased as HIV RNA rebounded. Further study is required to clarify the histopathologic significance of this finding.