Lymphocytic infiltration of the thyroid gland in patients with hyperthyroidism is associated with the presence of serum antithyroidal microsomal antibodies (TMA) and serum antithyroglobulin antibodies (TGA). The aim of this study was to evaluate the clinical significance of TMA and TGA during and after treatment of hyperthyroidism with antithyroidal drugs. One hundred and fifty-four hyperthyroid patients were treated for 18 months with methimazole and then followed up for 18 months or more (mean, 24.8 +/- 12.6 months). Patients were classified into three group. group I, patients negative for TGA and TMA before and during 18 months of treatment, group II patients positive for TMA but negative for TGA before and during 18 months treatment and group III patients who were positive for both TGA and TMA before and during treatment. The relapse rates after discontinuation of treatment in these group were 44.7% (17 of 38), 29% (18 of 62) and 11.1% (6 of 54), respectively. The value in group I was significantly higher than that in group III (P < 0.01). These results show that presence of TMA and TGA influence the prognosis of patients with hyperthyroidism treated with methimazole with regard to relapse. Those patients who had both antibodies were least likely to have a relapse and those who had neither antibody before and during treatment were most likely to have a relapse of hyperthyroidism.