BACKGROUND/AIMS: Interferons have been employed in the treatment of patients with hepatitis B and C. However, immunomodulatory properties of interferon are known to induce thyroid dysfunction. In the present study, we examined the prevalence of thyroid dysfunction and the development of thyroid autoantibodies following administration of interferon alpha (IFN-alpha), and investigated predictive fac tors that induced thyroid dysfunction. METHODS: Sixty-eight patients with chronic hepatitis B and 42 patients with chronic hepatitis C were investigated retrospectively. All of them were treated with IFN alpha. Before and after the treatment, FT3, FT4, TSH, antithyroglobulin antibody (Tg-Ab) and antimicro somal antibody (Ms-Ab) were measured. RESULTS: During treatment, thyroid dysfunction developed in 8 (7.3%) of the 110 patients. At the start of IFN-alphatherapy, 3 cases had detectable thyroid autoanti bodies. During IFN-alphatherapy, 6 additional cases became to have thyroid autoantibodies. Logistic regression analysis was performed for the variables such as age, gender, IFN dose and duration, HBV vs HCV, thyroid autoantibodies. It revealed that positivity of thyroid autoantibodies during and before therapy were statistically significant and independent risk factors for the development of thyroid diseases. CONCLUSIONS: IFN-alphatherapy in chronic viral hepatitis can induce thyroid dysfunction. All patients undergoing IFN-alpha therapy should be screened for thyroid autoantibodies and TFT before therapy and monitored during therapy. The occurrence of thyroid dysfunction requires a decision whether to maintain or interrupt IFN-alpha therapy, taking into account the response to therapy and theseverity of the thyroid disease.