Thyroid stimulating immunoglobulins (TSAb) were measured in fifty-four patients with Graves' disease before treatment with either radioiodine (seventeen patients) or propylthiouracil (PTU) (thirty-seven patients), and followed during treatment. After radioiodine TSAb increased to levels exceeding pretreatment values, and became detectable in three of six originally TSAb negative patients. In most patients TSAb decreased during treatment with PTU, and became undetectable after a mean of 12 months in patients above 40 years, and after a mean of 6 months in patients below 40 years. In order to eliminate the presumed causative agent in Graves' disease, antithyroid treatment should be at least 18 months in patients above 40 years, and at least 12 months in patients below 40 years of age. In twenty-nine patients TSAb were measured at cessation of 2 years antithyroid drug therapy. Ten patients were TSAb positive and all except one relapsed. Five of nineteen TSAb negative patients relapsed. Although TSAb positivity predict relapse, it is not an ideal index of prognosis after antithyroid therapy.