We have previously observed an increased frequency of liver failure in human immunodeficiency virus (HIV)-infected hemophiliacs. The purpose of this study was to quantitate hepatitis C virus (HCV) RNA levels in serial samples from HIV-seropositive (HIV+) and HIV-seronegative (HIV-) hemophiliacs before and after HIV seroconversion, and to examine the relationship of HCV RNA levels to CD4 cell counts and to hepatic dysfunction over time. HCV RNA levels were measured on serial samples of serum stored frozen from 17 HCV+/HIV+ and 17 HCV+/HIV- subjects matched within 5 years of their birth dates. All were HCV+ before study entry. HCV RNA levels were quantitated by a branched DNA-enhanced label amplification (bDNA) assay. For samples less than the cut off, HCV RNA was measured by the nested polymerase chain reaction. Individual changes over time, clinical groups, and mean values within predetermined time windows were compared with Wilcoxon rank sum tests. Mean HCV RNA levels increased from 2.76 (standard error [SE] 1.33) x 10(5) to 2.84 (SE 1.39) x 10(6) eq/mL during the first 2 years after HIV seroconversion (P = .006). Baseline HCV RNA levels in the pre-HIV seroconversion group were not significantly different from the baseline levels in those who remained HIV (P = .79). Over the entire period of study, HCV RNA levels increased nearly threefold in those who remained HIV- (mean 9.47 [SE 4.78] x 10(5) to 2.81 [SE 1.13] x 10(6)/mL; P = .02). Among those who became HIV+, HCV RNA levels increased 58-fold (mean 2.85 [SE 1.26] x 10(5) to 1.66 [SE 0.57] x 10(7) eq/mL; P = .0001). The rate of increase in HCV RNA levels was eightfold faster for HIV+ subjects than for subjects who remained HIV- (P = .009). HCV RNA levels increased twofold higher in 5 subjects who developed liver failure compared with the 12 who did not (P = .43). HCV RNA levels correlated significantly with CD4 counts (R = -.33, P = .01) and serum aspartate aminotransferase levels (AST) (R = .36, P = .007). We conclude that HCV RNA levels are significantly higher in HIV+ than in HIV- multitransfused hemophiliacs. HCV load increases over time, is enhanced by HIV, and further increases as immune deficiency progresses. HCV RNA levels are directly associated with high AST levels. These findings suggest that HIV-induced immune deficiency may promote increased HCV replication.