AIM: To evaluate the prevalence of autoantibodies in chronic hepatitis C virus (HCV)-infected children focusing on thyroid autoimmunity.METHODS: We investigated the prevalence of autoantibodies in 123 chronic HCV-infected children before,during and after monotherapy with interferon-alpha (TFN-α) or combined treatment with interferon-α or peginterferon-α and ribavirin. Besides antibodies against smooth muscle (SMA), nuclei (ANA), and liver/kidney microsomes (LKM), the incidence of antithyroid peroxidase antibodies as well as thyroid function parameters (TSH, FT3 and FT4) were determined.RESULTS: We found that 8% of children had autoantibodies before treatment. During treatment,18% of children were found positive for at least one autoantibody; 15.5% of children developed pathologic thyroid values during IFN-α treatment compared to only one child before therapy. Six children had to be substituted while developing laboratory signs of hypothyroidism.CONCLUSION: Our data indicate a strong correlation between interferon-α treatment and autoimmune phenomena, notably the emergence of thyroid antibodies. The fact that some children required hormone replacement underlines the need of close monitoring in particularly those who respond to therapy and have to be treated for more than 6 mo.