Adherence to hepatitis C virus (HCV) therapy has been incompletely examined among HIV-infected patients. We assessed changes in interferon and ribavirin adherence and evaluated the relationship between adherence and early (EVR) and sustained virologic response (SVR). We performed a cohort study among 333 HIV/HCV-coinfected patients who received pegylated interferon and ribavirin between 2001 and 2006 and had HCV RNA before and after treatment. Adherence was calculated over 12-week intervals using pharmacy refills. Mean interferon and ribavirin adherence declined 2.5 and 4.1 percentage points per 12-week interval, respectively. Among genotype 1/4 patients, EVR increased with higher ribavirin adherence, but this association was less strong for interferon. SVR among these patients was higher with increasing interferon and ribavirin adherence over the first, second, and third, but not fourth, 12-week intervals. Among HIV/HCV patients, EVR and SVR increased with higher interferon and ribavirin adherence. Adherence to both antivirals declined over time, but more so for ribavirin.