In this study the diagnostic value of ultrasound scanning of the thyroid is evaluated. Eighty-six patients with various thyroid disorders and 5 normal persons were investigated. In all patients and normal persons a 99mTc or 131I scintigram and an ultrasonic tomogram of the thyroid were made. Thyroid function also was assessed by a number of other laboratory methods. In 30 patients surgically removed thyroid specimens were examined pathologically. In addition to palpation and the scintigram, the ultrasonic tomogram gave good information about the size of the thyroid lobes and of thyroid nodules. Differentiation between a solid thyroid tumor and a cystic lesion was possible with this method. The value of diagnostic ultrasound in clinical thyroid investigation is discussed and several indications for its use are mentioned.