A 20-year-old woman given a diagnosis of hyperthyroidism (Basedow's disease) had been subsequently treated with methimazole since 1999. As she could not be made euthyroid, surgery was planned to relieve the symptoms. Because of liver dysfunction after discontinuation of methimazole and administration of iodine, she was admitted to the hospital. She was negative for hepatitis A, B and C virus serologies, but positive for anti-nuclear antibodies. A liver biopsy, which showed features of chronic active hepatitis, led to the diagnosis of autoimmune hepatitis (AIH). Interestingly, normalization of serum T4 correlated with improvement of serum aminotransferases. This leads us to speculate that this patient's liver dysfunction may have been AIH exacerbated by the liver dysfunction of hyperthyroidism rather than acute deterioration of AIH itself.