Hepatitis C virus infection is a common disease with a high propensity to progress towards chronicity. Alpha interferon has been proposed to halt progression of the disease and prevent the development of more severe liver diseases such as cirrhosis and hepatocellular carcinoma. Unfortunately, less than 20% of treated patients show a long-lasting ALT normalization and HCV-RNA negativity. Factors which might be predictive of a long term response to interferon are debated. Univariate analysis of data collected in randomized clinical studies, has shown that IFN dose, age, duration of disease, cirrhosis and early response (primary) to IFN were all predictors of a sustained response to IFN, but with differences in different studies. When data were analyzed by a multivariate analysis, short duration of the disease and absence of cirrhosis were found to predict long-term response to interferon. Finally, many evidences indicate that response rates to therapy may be better in patients with low pretreatment levels of HCV-RNA measured either by polymerase chain reaction (PCR) or the branched-DNA assay (b-DNA).