A patient without serologic evidence of an autoimmune disorder had a thyroidectomy for neck compression caused by her goiter. Histologic examination of the gland revealed pathologic changes compatible with Hashimoto disease, and lymphocytes isolated from her thyroid gland were transformed into lymphoblastoid cells in vitro by Epstein-Barr virus in the presence of cyclosporine. These cells produced antibodies against thyroglobulin, thyroid microsome, thyrocyte membrane, and thyrotropin, whereas lymphoblastoid cells produced from the patient's peripheral blood lymphocytes showed no antithyroid activity. Several human monoclonal antibodies were produced from the intrathyroidal lymphocytes by fusion with the SHM-D33 heteromyeloma, and the resulting cell lines produced antibodies with high-affinity binding specific for thyroid antigens. These studies show that lymphocytes capable of producing antithyroid antibodies can be present in the thyroid glands of patients with Hashimoto disease without evidence of a peripheral immune response, thus indicating that Hashimoto disease can exist as an organ-restricted autoimmune disorder.