Chronic viral hepatitis caused by hepatitis B, C or D may lead to cirrhosis, hepatocellular failure and hepatocellular carcinoma. The morbidity of these diseases has necessitated a prolonged search for effective therapy. Interferon-alpha has been studied widely, and remains the mainstay of treatment. Therapy for hepatitis B has now become possible with the demonstration that alpha-interferons inhibit hepatitis B virus (HBV) replication and that prolonged therapy can lead to a remission in disease. A number of other cytokines, including thymosin, are being evaluated. Currently used nucleoside analogues and anti-retroviral therapies used in human immunodeficiency virus infection have not proven useful in chronic hepatitis B. There are a number of new experimental nucleoside analogues with activity against HBV. Unfortunately, fialuridine has been associated with severe mitochondrial damage and hepatotoxicity. Other stereoisomers may be more active and less toxic, but the potential danger of these drugs indicates that large scale clinical trials should proceed cautiously. Experimental test systems for the preliminary investigation of antiviral compounds in hepatitis B and C will be required. Antisense oligodeoxyribonucleotides may inhibit the expression of the HBV genes. The natural history of hepatitis C is uncertain. Therapeutic trials of interferon-alpha indicated that a proportion of patients may respond to treatment with this agent. There is most information about 3 mU t.i.w. administered for 6 months. It is not yet clear whether this dose is optimal. Multivariate analysis of several pretreatment parameters indicate that patients without cirrhosis are more responsive to interferon. The influence of genotypes of hepatitis C is the subject of considerable interest at present. Patients with diverse circulating quasispecies may be less responsive to therapy than those with a single major species. Improved responses have been observed in patients with lower levels of circulating hepatitis C virus RNA.