Patients with autoimmune thyroid diseases, including Graves' disease and Hashimoto's disease, have marked lymphocytic infiltration in their thyroid glands. We examined the gene for the variable regions of the α-chain of the human T-cell receptor (the Vα gene) in intrathyroidal T cells to determine whether the infiltration is a secondary heterogeneous immune response or a more restricted, and therefore primary and presumably pathogenetic, reaction to thyroid autoantigens. We used the polymerase chain reaction to detect small numbers of T cells expressing the variable region of the Vα gene. Different oligonucleotides were used to amplify complementary DNA for the 18 known families of the Vα gene in intrathyroidal T cells from 9 patients with autoimmune thyroid disease. We compared the findings with the results in patients with nonautoimmune thyroid disease as well as those in normal subjects. We found marked restriction in the expression of T-cell–receptor Vα genes by T cells from the thyroid tissue of patients with autoimmune thyroid disease. An average of only 5 of the 18 Vα genes were expressed in such samples, as compared with 17 Vα genes expressed in peripheral-blood T cells from the same patients. No such restriction was found in thyroid tissue from patients with nonautoimmune thyroid disease. The predominantly expressed Vα genes differed from patient to patient, however, with no clear association with the type of disease. Intrathyroidal T-cell accumulation in autoimmune thyroid disease is highly restricted and points to the primacy of T cells in causing thyroid disorders. These results present the possibility of using antibodies to the T-cell receptor for the specific inhibition of abnormal T-cell function in autoimmune thyroid disease.