Thyroid microsomal (MAb) and thyroglobulin (TgAb) antibodies were sequentially measured by sensitive and quantitative radioassays in 17 patients with goitrous Hashimoto's thyroiditis (9 hypothyroid, 8 euthyroid) and in 19 patients with idiopathic myxedema before and at various time intervals up to 24-48 months after the institution of L-thyroxine therapy. Thyroid antibodies were also determined in 5 euthyroid subjects with Hashimoto's thyroiditis maintained without treatment for a similar period. During L-thyroxine administration a reduction of MAb with respect to the pretreatment level was found in 6 of the 9 (67%) hypothyroid patients with Hashimoto's thyroiditis and in 16 of the 19 (84%) patients with idiopathic myxedema. The decrease of MAb was highly significant in both groups (p less than 0.001 and p less than 0.0001, respectively). A fall of TgAb occurred in 2 of the 3 patients (75%) with hypothyroid Hashimoto's thyroiditis and in 9 of the 10 (90%, p less than 0.001) patients with idiopathic myxedema having abnormally elevated pretreatment TgAB levels. No consistent pattern of MAb and TgAb changes was observed in the euthyroid subjects with Hashimoto's thyroiditis, whether treated or untreated. In the hypothyroid patients with Hashimoto's thyroiditis a significant association was found between the decrease of MAb and the reduction of goiter size (p less than 0.05) occurring during L-thyroxine administration. Moreover, the decrease of MAb and TgAb in idiopathic myxedema was greater (p less than 0.05) in the patients with normalized serum TSH (less than or equal to 4 microU/ml) than in those showing only a partial reduction of serum TSH (greater than 4 microU/ml) under L-thyroxine.(ABSTRACT TRUNCATED AT 250 WORDS)