Two patients with hypothyroidism had detectable serum levels of thyrotropin binding inhibitor immunoglobulin (TBII). Patient 1 was a newborn infant who had transient neonatal hypothyroidism due to transfer of TBII from the mother with nongoitrous autoimmune thyroiditis. Patient 2 was an 8-year-old girl with Down's syndrome who presented with signs of myxedema and central precocious puberty. She had no goiter, and the recognition of thyroid disease was delayed; the histological diagnosis of chronic lymphocytic thyroiditis was established by aspiration biopsy, and TBII had strong thyroid adenyl cyclase-inhibiting activity in vitro. It appears that TBII may be pathogenetically important for occurrence of neonatal hypothyroidism and nongoitrous autoimmune thyroiditis without goiter.