Riedel's thyroiditis is an uncommon disorder of unknown etiology that is characterized by an invasive fibrotic process that partially destroys the gland and extends into adjacent neck structures. Its clinical manifestation as a stony-hard, poorly defined enlargement over the thyroid gland and local compression of the trachea, esophagus and recurrent laryngeal nerve can mimic invasive thyroid carcinoma. Because Riedel's thyroiditis is a self-limiting disease, its management should be conservative. However, invasive cancer such as follicular carcinoma can occur in association with Riedel's thyroiditis. Such a concurrence completely changes the focus of management. We report a case of Riedel's thyroiditis that was found in a patient with a follicular carcinoma. The strategy of management is discussed together with a review of the relevant literature.