Anti-adrenal and anti-pituitary autoantibodies have been determined in 45 patients with autoimmune diseases of the thyroid, including 25 patients with Graves' disease and 20 patients with hypothyroidism of autoimmune origin. The determinations were carried out with the use of solid-phase RIA methods previously developed by us, involving polyethylene tubes coated with the solubilized microsomal fractions obtained from human adrenal and pituitary glands. In the majority of patients with autoimmune diseases of the thyroid the presence of both anti-adrenal and anti-pituitary autoantibodies was detected. In 13 among 20 patients with hypothyroidism of autoimmune origin, the presence of anti-adrenal autoantibodies, and in 12 the presence of anti-pituitary autoantibodies was found. Among 25 patients with Graves' disease, 19 had both anti-adrenal and anti-pituitary autoantibodies. In the majority of patients with autoimmune diseases of the thyroid the titers of autoantibodies of both types were high but in no case were the clinical symptoms of adrenal or pituitary hypofunction observed. Our studies indicate that the thyroid diseases of autoimmune origin can be regarded as manifestation of some more generalized autoimmunization process.