A rare case of mucin-producing poorly differentiated adenocarcinoma of the thyroid is reported in a 58-year-old man. Light microscopically, the tumor composed of columnar cells, cuboidal cells and spindle cells and these tumor cells showed a various growth pattern; glandular growth with columnar cells, microfollicular growth with cuboidal cells and solid growth with spindle cells. Mucin histochemistry showed scattered foci of mucin production. Immunohistochemistry showed a focal positive staining for thyroglobulin and carcinoembryonic antigen, but a negative staining for calcitonin throughout the tumor. The patient had cervical and axillary lymph node recurrences after the radical operation. The histologic finding of this tumor is peculiar; we report this case briefly.