A case of malignant thymoma presenting as an anterior neck mass is reported. The tumor extended from the thyroid gland to the superior mediastinum. It did not accumulate Tc-99m pertechnetate, but continued to accumulate Tl-201 at the late phase. A fine-needle aspiration cytology from the tumor showed tight clusters of epithelial cells with crowded ovoid nuclei. The tumor was initially diagnosed as thyroid carcinoma, clinically and cytologically. A thymoma with a dominant epithelial component has to be considered in the differential diagnosis of a suspected papillary carcinoma of the thyroid.