Two sisters with nongoitrous cretinism, offspring of a consanguineous marriage, were found to have isolated thyrotropin (TSH) deficiency. Growth hormone, ACTH and gonadotropin secretion were estimated as intact. Serum TSH determined by radioimmunoassay was undetectable while these patients were hypothyroid, whereas it was very high in seven control patients with cretinism. Recovery and dilution experiments, and a prolonged half-life of ¹³¹I-TSH in blood, excluded the presence of an inhibitor or accelerated degradation of endogenous TSH. No increase in serum TSH was observed after administration of thyroid hormone or antithyroid drug. Administration of synthetic thyrotropin-releasing hormone failed to stimulate a rise in serum TSH levels. The uptake of radioiodine by the thyroid gland was appreciably increased by exogenous TSH.