Circulating thyroglobulin antibodies (TgAb), microsomal antibodies (MsAb) and thyroid function (total and free T4 [thyroxine] and T3 [triiodothyronine], TSH basal and after TRH) were evaluated in 92 hyperprolactinemic patients (82 females and 10 males; 9 with macroprolactinoma, 22 with microprolactinoma, 4 with acromegaly, 5 with organic lesions of the hypothalamus, 2 with empty sella, 2 with idiopathic hypopituitarism, 2 with primary hypothyroidism and 46 with idiopathic hyperprolactinemia). Thyroid function was normal in all cases except 3 with hypothalamic disease and central hypothyroidism, the 2 patients with primary hypothyroidism and 2 with thyrotoxicosis (one due to Graves'' disease and one to autonomous thyroid adenoma). High titers of TgAb (.gtoreq. 1/1250) and/or MsAb (.gtoreq. 1/1600) were found in the subject with Graves'' disease, in 1 acromegalic, in the 2 primary hypothyroids, and in 12 women with either adenomatous or idiopathic hyperprolactinemia; low titers of one or both antibodies were found in 9 other euthyroid women and in the one with toxic adenoma. In a control population of 185 subjects studied with the same methods, the prevalence of TgAb and/or MsAb positive (low titers) was 3.3% in females and 2.5% in males. Diffuse thyroid hyperplasia was clinically detectable in 12 euthyroid women and in the one with Graves'' disease; 3 others had been previously operated for nodular goiter with histological evidence of Hashimoto''s thyroiditis (2 cases) or for a cold nodule; a single thyroid nodule was present in the woman with toxic adenoma and in 1 euthyroid woman. Most of these subjects also had circulating TgAb and/or MsAb, and a few had increased TSH secretion. No significant differences were found in mean thyroid hormone and TSH levels between euthyroid hyperprolactinemic subjects and healthy controls, but TRH-simulated TSH levels were significantly higher in thyroid antibodies positive than negative subjects. These data, in agreement with a few previous reports, suggest that autoimmune thyroid disorders (especially asymptomatic autoimmune thyroiditis) occur in hyperprolactinemic women with a prevalence for exceeding that observed in many surveys in the general population.