The results of treatment were analyzed in relation to serum microsomal antibody (MCAb) titre before treatment in 1185 patients with Graves' disease. The percentage of patients who had ablative therapy because of poor response to antithyroid drug treatment was significantly greater in those with MCAb haemagglutination test (MCHA) titres greater than 1:25,000. With 131I treatment, the patients with MCHA titres greater than 1:6400 responded significantly less to therapy, although the analysis was done in 146 selected patients with certain defined radiation doses and small goitres. With surgical treatment, the percentage of the patients entering into remission was significantly smaller for patients with MCHA titres greater than 1:25,000, because of an increase in both hypothyroidism and relapses. The incidence of hypothyroidism was significantly higher in patients with marked lymphocyte infiltration and/or lymphoid follicles. The degree of these histological findings in Graves' disease was not marked in spite of high MCAb titre and it was significantly different from that in Hashimoto's disease when analyzed in relation to the MCHA titre. These data indicate that in Graves' patients with high MCAb titre, remission is difficult to obtain by treatment, and suggest that the significance of MCAb is different in Graves' disease and Hashimoto's disease. The titre in Graves' disease may be one expression of the activity of this disease.