Previous comprehensive biochemical studies of thyroidal function in cretins were each performed on a few individuals. Such studies do not permit an evaluation of the incidence of various thyroidal defects in cretins. We have therefore performed a comprehensive clinical and biochemical study of 56 cretins from the State of Michigan, including 41 born after the introduction of iodized salt into Michigan. Cretins were considered to have a thyroid gland if it was palpable, was found surgically, demonstrated by scintigram, or showed a net radioactive iodine (I131) uptake over the thyroidal area exceeding 10%, or if there was evidence of incorporation of administered I131 into circulating compounds. The minimum incidence of thyroid glands in cretins was 21% and the probable incidence 32%. Inability of the thyroid gland to organify iodine was detected by discharge of I131 after administration of potassium thiocyanate (KSCN) to six patients, and chromatography of the thyroid gland hydrolysates in four patients. Inability to organify iodine was the most common clearly identifiable defect in these cretins with thyroid glands. The familial incidence of cretinism was higher in subjects with thyroid glands than in athyrotic subjects. Cretins with thyroid glands co-existed in the same sibship with athyrotic cretins. Serial studies of thyroidal function indicate that progressive postnatal failure of thyroid function may result in athyreosis.