To evaluate the safety and efficacy of an induction dose of pegylated interferon alpha 2a (IFN-alpha2a) on the 12-week hepatitis C virus (HCV) kinetics in human immunodeficiency virus (HIV) patients co-infected with HCV. One hundred sixteen HIV/HCV co-infected patients from nine hospitals in Spain were randomized to receive 270 microg/week of pegylated IFN-alpha2a for 4 weeks followed by 180 microg/week for 8 weeks or 180 microg/week for 12 weeks. Ribavirin was given at a daily dose of 1000 or 1200 mg. The main outcome measure was the percentage of patients achieving an HCV-RNA below 50 IU/mL or a decrease of 2 or more log(10) at week 12 (early virologic response, EVR). HCV-RNA was measured at baseline, weekly, for the first 4 weeks and monthly thereafter. We observed no difference in the percentage of patients achieving an EVR between arms (on-treatment, 74% in both arms; intention-to-treat, 70% in the induction arm and 67% in the control arm), nor were there differences in the percentage achieving an undetectable HCV qualitative polymerase chain reaction at any time points or in the decrease in HCV-RNA from baseline. No differences were found between arms in the percentage of dropouts (8% in the whole study population). Our study failed to find a benefit of an induction dose of 270 microg/week of pegylated IFN-alpha2a for 4 weeks on the EVR in co-infected patients who are treatment naive. Despite the lack of benefit with this regimen, induction therapy with this schedule was safe and well tolerated in co-infected patients.