Antithyroid drugs have been reported to reduce the expression of HLA-DR in thyrocytes in Graves' disease, but only circumstantial evidence has been provided about their in vivo immunologic effects. This present study was designed to examine the in vivo immunologic effect of antithyroid drugs on thyroid follicular cells. The study was conducted on 25 patients who had Graves' disease in remission or in activity and who were or were not receiving treatment (7 in overt thyrotoxicosis, 6 patients in remission, and 12 patients under medication). HLA-DR expression in thyroid biopsies was verified by immunohistochemistry. The follicular cells of all patients in overt thyrotoxicosis expressed HLA-DR whereas those of patients in remission were negative for HLA-DR. HLA-DR was also not expressed in all patients under medication, but this did not correlate with the clinical evolution after thyroid drug withdrawal. In conclusion, antithyroid drugs inhibit follicular cell HLA-DR expression in Graves' disease, when thyrotoxicosis is controlled. This suggests that additional mechanisms not involving HLA-DR play a role in thyroid autoimmune disease.11575-80