Screening for antibodies to hepatitis C virus (HCV) has substantially reduced the risk of HCV infection from transfusion of blood and blood products. Nevertheless, new infections may continue to occur. These infections may be caused by donor infections that escape detection or by insufficient decontamination of blood products during preparation. Frequently, HCV infection becomes chronic, is clinically silent, and can be associated with extrahepatic illnesses and liver cancer. A recent outbreak of HCV infection in patients who received intravenous immunoglobulin has permitted study of this infection in immunocompromised hosts. Some evidence indicates that this infection is more virulent in these patients, and our experience at Children's Hospital, Boston, Massachusetts, substantiates this finding. In this article, epidemiology and modes of transmission of HCV, pathogenesis of HCV infections, differential diagnosis, and clinical features of HCV infection in both children and adults are discussed. Particular attention is given to the serologic findings seen in the spectrum of diseases associated with chronic HCV infection. The mechanisms of action of interferon alfa treatment of HCV infections are discussed. However, results of interferon alfa treatment of patients with HCV infection at Children's Hospital are preliminary.