Hepatitis C virus (HCV) is now the most common cause of death in patients coinfected with the human immunodeficiency virus (HIV). In the U.S., 1/3 of all HIV-infected patients are coinfected with HCV. HIV/HCV coinfection has been associated with rapid progression of liver fibrosis, cirrhosis, and hepatic failure. Thus, HCV therapy is critical for long-term survival of HIV/HCV-coinfected patients. However, -70% of HIV/HCV patients have been reported to develop severe weight loss during treatment of HCV, and this can result in premature cessation or modification of HCV therapy. Since this adverse effect can prevent patients at high- risk of advanced liver disease from receiving optimal therapy, determination of the incidence and degree of weight loss among HIV/HCV patients receiving dual HIV/HCV therapy is needed. Studies that also elucidate risk factors for weight loss can enhance understanding of biological mechanisms behind it. This retrospective cohort study seeks to examine the incidence of weight loss in dually treated HIV/HCV patients compared to 1) HCV-monoinfected patients who receive HCV therapy and 2) HIV-monoinfected individuals receiving only HIV therapy. Risk factors for weight loss will also be examined. The results of this epidemiological study may ultimately help to allow optimal treatment of HCV in HIV/HCV patients and provide the foundation to evaluate changes in body composition and metabolic abnormalities accompanying weight loss. [unreadable] [unreadable]