Background: The combined regimen of peg-interferon α2a plus ribavirin was considered as an effective therapy for hepatitis C infections. However, this combined therapy has been associated with a side effect that can make the course of the disease more complicated. The aim of this study is to quantify the risk of new-onset diabetes mellitus among patients who ontreatment and who finished the standard of care treatment in a large cohort of patients with chronic hepatitis C. Materials and Methods: We used prospectively obtained data for 600 HCV-infected patients who finished their standard of care treatment within the previous one year, who were free of diabetes at baseline; diabetes is a study end point. Incidence of new-onset diabetes and impaired fasting glucose were assessed at follow-up at 6 months and 12 months. Baseline data for body-mass index (BMI) and other risk factors were recorded at baseline time. Results: During the one year period of follow-up of the 600 HCV-infected patients who finished their standard of care treatment, 53 patients developed new-onset diabetes (8.9%) and 50 patients developed new-onset IFG (8.3%). Of the 53 patients who developed DM: 11 (1.8%) patients developed DM with the end of treatment, 22 (3.6%) patients developed DM at 6 months after the end of treatment, and 20 (3.5%) patients developed DM at one year of the end of treatment Conclusion: The treated HCV patients have a higher incidence of new onset diabetes mellitus than the general population (8.9% versus < 0.1% per year).