Autoantibodies to thyroid peroxidase (TPO) predominately recognise conformational epitopes, restricted to two immunodominant regions (IDR) -A and -B. We have estimated the proportion of IDR-A and -B autoantibodies in 75 Hashimoto and 68 Graves patients sera. There were no statically significant differences between Hashimoto and Graves patients sera in the IDR-A and -B autoantibodies level, despite great differences between individual patients sera. In 75 Hashimoto patients sera, the mean value of IDR-A was 24.26%, IDR-B--52.26%, IDR (A + B)-76.66%, and to non-A, non-B regions--23.5%. In 68 Graves disease patients, the mean value of IDR-A was 24.87, IDR-B--54.29, IDR (A + B)--79.07, non-A, non-B--20.92%, at a single autoantibodies concentration 5IU/ml. At high autoantibodies concentration (50 IU/ml) the proportion of autoantibodies to the IDR (A + B) diminished to 58% and to the non-A, non-B TPO regions increased to 42%. The autoimmune response to TPO regions outside the IDR (A + B) epitopes is stronger then previously assumed and this response is also conformation dependent.