To assess the effect of human immunodeficiency virus (HIV) immunosuppression on ongoing hepatitis C virus (HCV) infection, CD4 lymphocyte counts and serum concentrations of HCV RNA, HIV RNA, and alanine aminotransferase (ALT) were evaluated among members of a cohort of injecting drug users (IDUs). With 100 participants randomly selected at various stages of HIV-related immunosuppression, serum HCV RNA concentrations increased with age (P = .007) and were higher in HIV-positive IDUs with 201-500 (P = .026) and 51-200 (P = .004) CD4 cells/mL than in HIV-negative participants. Among 27 HCV-infected IDUs who acquired HIV infection, serum HCV RNA concentrations varied between semiannual visits by a mean of 0.45 logs, increasing by 0.60 logs after HIV seroconversion (P < .0001), by 0.12 logs each subsequent year (P = .006), and by 0.36 logs per log increase in CD4 cells (P = .01). Serum ALT levels were similar between HIV-positive (40.1 IU/mL) and HIV-negative (45.4 IU/mL) patients (P > .10). While HIV infection and possibly HIV progression are associated with increased HCV RNA levels, other factors appear to affect biochemical and virologic markers of HCV infection in some dually infected persons.