A 41-year-old hyperthyroid woman visited the hospital in April 1989. She spontaneously developed hypothyroidism in August 1989 and recurrent hyperthyroidism in November 1991. TSH binding inhibitor immunoglobulins (TBII) and thyroid-stimulating antibodies (TSAb) activities were 34.7% (normal range, <11.0%) and 295.0% (normal range, <145.0%), respectively, at her first visit. When she was hypothyroid, TBII were 85.9-90.0%, TSAb were 75-190%, and thyroid stimulation-blocking antibodies (TSBAb) activities were 80.0-96.6% (normal range, <32.2%). When she became hyperthyroid again, TSAb activities were markedly increased to 1108% with TBII and TSBAb activities being 82.2 and 62.8%, respectively. The results indicate that both stimulating and blocking types of TSH receptor antibodies were present in her serum and that her fluctuating thyroid function could be explained by alterations in the balance between the activities of these two types of antibodies.