Hepatitis C virus (HCV) is an RNA virus of the Flaviviridae family and is a major cause of chronic hepatitis, cirrhosis, and hepatocellular carcinoma. Owing to shared routes of transmission, HCV and human immunodeficiency virus (HIV) coinfection are common, affecting approximately one-third of all HIV-infected persons in the United States. In addition, HIV coinfection is associated with higher HCV RNA level and a more rapid progression of HCV-related liver disease, which leads to an increased risk of cirrhosis. HCV infection may also impact the course and management of HIV disease, particularly by increasing the risk of antiretroviral drug-induced hepatotoxicity. Thus, chronic HCV infection acts as an opportunistic disease in HIV-infected persons, because the incidence of infection is increased and the natural history of HCV infection is accelerated in coinfected persons. Strategies to prevent primary HCV infection and to modify the progression of HCV-related liver disease are urgently needed for HIV-HCV-coinfected individuals.