The degree of adherence to anti-hepatitis C virus (HCV) therapy among HIV/HCV-coinfected patients is not known. A prospective cohort study was performed in two groups of patients: 79 HIV/HCV-coinfected patients (group 1) and 78-HCV-monoinfected patients (group 2). Patients were treated with interferon alpha-2a (3 million international units [MIU], three times per week) plus ribavirin (1000-1200 mg/day) for 48 weeks. Adherence to therapy was defined as having received +/-80% of both drug dosages for +/-80% of the expected duration of therapy. The degree of adherence to treatment was similar for patients with or without HIV coinfection (72.2 versus 80.8%). The overall sustained virological response (SVR) in patients with adherence to therapy was 41.7% as compared with only 8.1% (p = 0.0001) in patients without adherence. The difference in SVR rate according to adherence to treatment was also evident in patients of group 1 (29.8% versus 9.1%; p = 0.05) as well as in those of group 2 (52.4 versus 6.7%; p = 0.001). Adherence to anti-HCV therapy, which can be similar in mono- and coinfected patients, enhances the likelihood of achieving an increase in SVR rate. In addition to improved adherence, in coinfected patients more aggressive therapeutic strategies may be necessary to achieve SVR.