Chronic hepatitis C virus infection is a common and serious disease. Although an estimated 2.7 million persons in the United States have this disease, most have not yet been diagnosed. Recent advances in treatment provide successful cure in 50 to 80 percent of cases. Current drug therapy consists of a combination of pegylated interferon and ribavirin. Although all patients with chronic hepatitis C virus infection are potential candidates for treatment, pharmacologic therapy has a number of contraindications. Evaluation of suitability for treatment includes a thorough search for comorbid medical and psychiatric conditions that can be contraindications. Initial testing involves anti-hepatitis C virus antibodies, but definitive diagnosis of active disease requires detection of viral RNA. Most patients require a liver biopsy to determine the amount of hepatic fibrosis and ongoing hepatocellular inflammation. Viral genotype also should be determined: type 1 requires 12 months of treatment and does not respond as well as types 2 and 3, which require only six months of treatment. Common side effects of drug therapy include anemia, anorexia, depression, fatigue, fever, headache, myalgia, nausea, and erythema at the injection site.