Abstract Two patients with congenital hypothyroidism and thyroid-stimulating hormone-secreting pituitary adenomas are described. Both patients had a history of long-standing, inadequately treated hypothyroidism accompanied by markedly elevated levels of serum thyroid-stimulating hormone. Pituitary tissue, obtained at autopsy In the first patient and at surgery in the second, disclosed a thyrotropic adenoma. Immunocytochemlcal studies with anti-thyroid-stimulating hormone antisera showed both tumors to be strongly immunoreactive. Such thyrotropic adenomas of the pituitary gland are presumed to occur as the result of protracted pituitary stimulation secondary to long-standing thyroid deficiency.