In a 68-year-old Japanese woman, who had a right supraclavicular cyst containng thyroglobulin-rich fluid, metastatic adenocarcinoma was demonstrated in the cyst. A subsequent total thyroidectomy showed a papillary carcinoma measuring 11 mm in the ipsilateral lobe of the thyroid. A high thyroglobulin level in a cervical cyst may facilitate the detection of hidden primary cancer in the thyroid.