The high worldwide prevalence of chronic viral hepatitis, as well as its progressive course, have led to the performance of multiple clinical studies. The natural history of the disease is different depending on the infecting virus; thus, the evolution to liver cirrhosis and/or hepatocellular carcinoma for the hepatitis B, C and delta (D) viruses in chronic hepatitis is 15, 20 and 75%, respectively. Different therapeutic agents have been used in attempts to modify the natural course of these diseases, interferon-alpha (IFN alpha) having proved to be the most effective. In 30 to 50% of patients, treatment with IFN alpha has achieved inhibition of viral replication, as well as normalisation of aminotransferase levels. Moreover, in a majority of patients, histological improvement is observed, principally in piece-meal necrosis and portal inflammation. The dosage currently recommended for treatment of chronic hepatitis B is 30 to 35MU weekly for a minimum of 4 months; when there is a co-existing delta virus infection, the total dosage employed should be greater. For hepatitis C, the minimum effective dosage is 9MU weekly, and a treatment duration of 12 months is recommended. The administration of IFN alpha produces a series of dose-dependent adverse effects, which are reversible on suspension of the medication. The most frequent of these adverse reactions is the 'flu-like' syndrome, which is self-limited and generally well tolerated. Secondary haematological alterations (leucopenia and thrombocytopenia) are the most frequent cause of reduction in dosage or suspension of treatment, although the latter is not normally necessary. The proportion of patients requiring dosage modification or suspension of treatment fluctuates between 5 and 15%. Taking the evolution of chronic hepatitis into account, there can be no doubt that all patients with this disease should be offered treatment. At present, the drug of choice is IFN alpha, as it slows disease progression and it is generally well tolerated.