: OBJECTIVES: The authors described a case of Riedel's thyroiditis, a relatively rare disease affecting the thyroid gland. RESULTS: A 38-year-old female was diagnosed with Riedel's thyroiditis by a frozen section examination during attempted surgery for the suspicion of malignancy. The histopathologic examination confirmed the diagnosis of Riedel's thyroiditis combined with signs of giant cell arteritis. After hemithyroidectomy, the patient had paresis of the recurrent laryngeal nerve for five days and signs of hypothyroidism and hypoparathyroidism for three months after surgery. The level of thyroid peroxidase antibodies titer was higher. The patient had no compression symptoms, no signs of systemic immunopathology, and no signs of extracervical fibrosis. She was put on prednison 5 mg daily. CONCLUSIONS: Riedel's thyroiditis can be looked upon as a cervical fibrosis of unknown etiology affecting the thyroid. In our case it was combined with signs of giant cell arteritis.