In order to assess the relationship between human immunodeficiency virus (HIV) RNA, hepatitis C virus (HCV) RNA, CD4, CD8, and liver enzymes during combination antiretroviral therapy, these parameters were measured in 12 HIV-HCV-coinfected patients (who were naive for antiretrovirals) on the day before and 3, 7, 14, 28, 56, and 84 days after initiating the following treatments: stavudine and lamivudine in all patients, indinavir in 6 patients, and nevirapine in 6 patients. HIV RNA declined rapidly, CD4 cells increased slowly, and CD8 cells and liver enzymes were stable. HCV RNA showed a transient significant increase at days 14 and 21 (7.33+/-0.16 [mean +/- SE] and 7.29+/-0.2 log copies/mL vs. 7+/-0.2 log copies/mL at baseline; P<.05). These changes were similar in both treatment groups. A 2-fold alanine aminotransferase increase was observed in 4 of 12 patients; 4 of 4 patients showed increased HCV RNA. The relationship between HCV RNA increase and HIV RNA decrease indicates virus-virus interference. An HCV RNA increase may cause significant liver damage only in a minority of patients.