Hepatitis C virus (HCV) is responsible for most cases of parenterally transmitted non-A, non-B hepatitis; it is also a cause of sporadic acute hepatitis. Hepatitis C viral infection becomes chronic in 50% of cases; 25% of these patients will develop cirrhosis. Therapy with interferon results in approximately 25% response rates (after subtraction of relapse rates), and probably the same reduction in morbidity of cirrhosis.