Thyroglobulin antibodies (TgAbs) are typically found in autoimmune thyroid diseases and, more rarely, in nonautoimmunc thyroid diseases and healthy subjects. To determine whether TgAbs associated with different conditions recognize different epitopes on the thyroglobulin molecule, we studied 28 patients with Hashimoto's thyroiditis, 30 with Graves' disease, 21 with thyroid carcinoma, 18 with nontoxic goiter, and 25 healthy subjects. All patients were selected for the presence of TgAbs; 4/25 healthy subjects also had TgAbs. The sera were assayed for the their ability to inhibit the binding of monoclonal antibodies to thyroglobulin in an EL1SA assay. We found that: 1) TgAbs in Hashimoto's patients preferentially recognized three clusters of epitopes (II, III and typically VI). with no difference between the goitrous and the atrophic variants; 2) TgAbs in Graves' patients were directed toward cluster II, with no difference between the presence or the absence of ophthalmopathy; 3) TgAbs in thyroid carcinoma patients recognized the same clusters as Hashimoto's patients; 4) TgAbs in nontoxic goiter patients and in the four healthy subjects showed no restriction in epitope recognition. We suggest that in individuals with no overt clinical or biochemical thyroid abnormalities but with TgAbs, the finding that these TgAbs recognize particular immunodominant clusters may be utilized to predict full-blown thyroid disorders. Longitudinal studies are needed to evaluate the possible clinical application of this methodology.