Background The Optimized Pegylated interferons Efficacy and anti-Retroviral Approach (OPERA) study aimed to assess the efficacy and safety profile of treatment with pegylated interferons (PEG-IFNs) in interferon-naive patients with chronic HCV and HIV infection in routine clinical practice. Methods This was a multicentre, prospective observational cohort study conducted at 98 Italian referral centres for the treatment of chronic HCV and HIV coinfection. Adult subjects ( n=1,523) with a confirmed diagnosis of HCV and stable HIV coinfection were followed between April 2005 and March 2011; of these, 1,284 were interferon-naive and were the focus of this analysis. Patients received PEG-IFN-α2a or -α2b plus ribavirin combination therapy. The choice of treatment and dose was at the investigator's discretion, according to the summary of product characteristics and current guidelines. The primary efficacy end point was sustained virological response (SVR). Secondary end points included rates of rapid viral response, early viral response and response at end of treatment. Results SVR was achieved by 40.0% of patients; the highest SVR rate was observed in patients with HCV genotypes 2 and 3. More genotype 2 and 3 than genotype 1 and 4 patients achieved rapid and early viral responses, and end of treatment responses. Higher SVR rates were also associated with ≥80% anti-HCV treatment compliance and lower baseline HCV levels. Conclusions The OPERA study results show that PEG-IFN plus ribavirin is an effective treatment for HCV–HIV coinfection in interferon-naive patients. Independent predictors of SVR include HCV genotype, undetectable baseline HIV RNA and baseline HCV RNA<500,000 IU/ml.