The effect of treatment for hepatitis C viral infection on hemoglobin A(1c) (A1C) levels is not well described in the literature. We describe a 59-year-old man with type 2 diabetes mellitus whose A1C level became falsely low when ribavirin and peginterferon alfa-2b therapy were started for treatment of hepatitis C. After treatment was discontinued, the patient's A1C returned to its previous baseline value. Use of the Naranjo adverse drug reaction probability scale indicated a probable relationship (score of 7) between the patient's low AIC level and his ribavirin-peginterferon alfa-2b therapy. Clinicians should be aware that combination therapy for hepatitis C may affect A1C values. To maintain accurate glucose control in patients with diabetes who are receiving treatment for hepatitis C, it is important that they self-monitor their blood glucose levels in conjunction with A1C data, especially when A1C values become falsely low.