Demonstration of ectopic thyroid tissue is diagnostic of thyroid dysgenesis. Delineating the lingual thyroid is, therefore, one of the important issues in ultrasonic imaging. We delineated the lingual thyroid in eight patients with congenital hypothyroidism using two basic scanning positions: the midline sagittal and the posterior coronal views of the floor of the mouth. All the lingual thyroids were located close to the hyoid bone. This location correlated well with the observations on radioisotope scanning. We emphasise the importance of a knowledge of the sonographic anatomy of the floor of the mouth in obtaining satisfactory results in this examination.