We examined the effects of anti-thyroid drug treatment on serum autoantibodies against thyroid hormones (thyroid hormone autoantibodies, THAA), thyroglobulin (Tg) and thyroid peroxidase (TPO) in patients with Graves' disease by measuring each autoantibody level before and after treatment. Six patients among 40 untreated patients with Graves' disease had anti-thyroxine (T4) antibodies. One patient had both anti-T4 and anti-triiodothyronine (T3) antibodies. Thus the prevalence of THAA in untreated Graves' disease was 7 out of 40 (17.5%). Changes in T4-Ab levels after treatment varied. In five cases (cases 3-7) levels decreased 4-7 months after treatment. However, in the other two cases levels fluctuated 1, 3, 6 and 12 months after treatment. None of the previously THAA-negative patients became positive after treatment. Anti-Tg antibody (Tg-Ab) was positive in 34 out of 40 (85%) untreated cases and its level decreased in both THAA positive and negative patients after treatment. Anti-thyroid peroxidase antibody (TPO-Ab) was positive in 32 of the 40 (80%) untreated Graves' patients and its level significantly decreased after treatment. Our findings suggest that treatment with anti-thyroid drugs does not produce THAA in Graves' disease.