Ultrasound was used to study 10 patients with subacute thyroiditis. With gray-scale technique, a "washed-out" appearance was seen during the active phase of the active phase of the illness. Clinical migration of the inflammation was accompanied by migration of the sonographic abnormality. Ultrasound can be especially helpful in the diagnosis of unilateral subacute thyroiditis and the differentiation of true cysts from hemorrhagic degeneration of goiter.