A 58-year-old woman with thyroid hemiagenesis associated with subacute thyroiditis is described. Physical examinations revealed a firm nodule with spontaneous pain and tenderness in the left thyroid lobe. A serum thyroid hormone levels were elevated and thyroid scintiscan with 99mTc pertechnetate showed an extremely low uptake, we made a diagnosis of subacute thyroiditis. A 201Tl thyroid scan demonstrated an enlarged left lobe and absence of the right lobe. Ultrasonography of the thyroid gland revealed an enlarged left lobe occupied mostly with a hypoechoic region. An incidental finding was absence of the right lobe. Two months later thyroid function returned to normal. A follow-up thyroid scintiscan revealed a normal left lobe but absent right lobe. The diagnosis of hemiagenesis was confirmed by CT scan. This appears to be the first case report of thyroid hemiagenesis associated with subacute thyroiditis.