This study was carried out to investigate the fate of the ultimobranchial contribution to the thyroid in man when the thyroid lobe is not fully descended. Thyroid abnormalities were recorded in 41 of 29,000 autopsies at the London Hospital--a prevalence of 1.41/1000. The 18 major abnormalities included four cases of lingual thyroid, in two of which cystic structures were noted in the neck in the region of the upper parathyroid. Three further cases of lingual thyroid with upper parathyroid cysts were collected from other sources. The multilobulated cystic structures were up to 1.5 cm in diameter and showed a variety of histologic features. In four cases there were glandular nodules of up to 1 mm across lying in the intercystic connective tissue. These nodules showed solid areas of irregularly distributed cells that stained positively for calcitonin and calcitonin gene-related peptide, intermingled with, and sometimes apposed to, follicular structures that showed follicular cells and colloid that stained for thyroglobulin. This study has provided conclusive evidence that the ultimobranchial body contributes both C cells and follicular cells to the thyroid in man, has documented the prevalence of congenital defects of thyroid development, and has described the human ultimobranchial gland as showing a wide developmental potential which may have implications in the histogenesis of thyroid neoplasia.