Interferon is the only treatment shown to be effective on hepatitis C in controlled trials. The response to treatment is generally assessed in terms of a return to normal transaminase activity, but also negative PCR testing for viral RNA and histopathological examination of the liver. At a dose of 3 MU three times a week for 6 months, 25% of patients have a persistent return to normal transaminase activity, 25% relapse when interferon is withdrawn, and the remaining 50% have persistently high levels at the end of treatment and are considered resistant. The rate of persistent responses increases to 40% when treatment is extended to one year. Viral RNA becomes undetectable in the serum of 80% of these responders. Most also have a histological improvement, but so do a number of patients who relapse or who are resistant. In the longer term, interferon could prevent the onset of liver cancer in patients with viral C cirrhosis. Interferon is generally well tolerated at the doses currently used, most side effects (hematologic, neuropsychiatric and thyroid disorders) resolving when treatment is stopped. The following factors are clearly predictive of the response to interferon : young age, short time since onset, absence of cirrhosis, lower-level viremia, and infection by HCV genotypes other than 1b. Interferon is markedly less effective in immunodeficient patients (transplant, HIV infection, etc.). Several add-on treatments have been tried, but ribavirin appears to be the most promising, both during initial interferon therapy and for patients who relapse or are resistant to a first course. Interferon therapy of the acute phase of hepatitis C significantly reduces the risk of chronic liver disease. There is no vaccine against HCV infection.