The presence of serum antithyroglobulin (TGHA) and antithyroidal microsomal (MCHA) antibodies in Graves' disease patients is associated with lymphocytic infiltration of the thyroid. The aim of this study was to determine the clinical significance of TGHA and MCHA during and after treatment of hyperthyroidism due to Graves' disease. One hundred and seventeen such patients were treated for 2 yr with methimazole and then followed for an additional year or more (mean, 30 months). The patients were classified into the following three groups: group I, patients negative for TGHA and MCHA before and during the 2 yr of treatment; group II, patients positive for MCHA but negative for TGHA before and during the 2 yr of treatment; and group III, patients who were positive for both TGHA and MCHA before and during treatment. The relapse rates after discontinuation of treatment in these groups were 39% (13 of 33), 27% (13 of 48), and 11% (4 of 36), respectively; the value in group I was significantly higher than that in group III (P less than 0.01). The results suggest that the presence of TGHA and MCHA may influence the prognosis of Graves' disease in patients treated with methimazole. Those patients who had neither antibody before and during treatment were most likely to have a relapse of hyperthyroidism, and those who had both antibodies were least likely to have a relapse.