Thyroid-stimulating antibodies (TSAb) were studied in patients with Graves' disease using a method based on cAMP production in isolated human thyroid membranes. Stimulation was detected in forty-one (82%) of fifty patients with untreated Graves' disease. In these subjects, the TSAb levels were correlated with the thyroid hormone levels. Among twenty patients treated for 1-2 months with carbimazole, 16 (80%) had positive TSAb. During prolonged treatment TSAb gradually diminished and finally normalized. In fifteen patients, it was possible to compare TSAb levels after cessation of previous medical therapy with TSAb levels at relapse. In nine of these patients, an increase of the TSAb level within the normal range at the time of relapse was found, in four the titres were positive. The results indicate that positive TSAb titres are markers of active Graves' disease and suggest that in such patients antithyroid therapy should be continued. A normal TSAb titre after anti-thyroid therapy does not exclude the possibility of relapse.