A 47-year-old woman consulted us about a painless breast mass. It was 3.5×4.5cm in diameters, and lo-cated in the upper-outer quadrant of the right breast. It was elastic and hard and well movable. Axillary lymph nodes were not palpable. Mammography showed a dense mass.
Ultrasonography revealed a mass with an unclear contour. Halsted's mastectomy was performed immediately after an intraoperative diagnosis of carcinoma. Histopathological examination revealed a medullary carcinoma with abundant mucin production. Estrogen receptor or progesterone receptor were negative. No metastatic node was found. Carmofur was prescribed postoperatively. The patient is alive without recurrence at 4 years and 8 months after mastectomy. We report a case of medullary carcinoma with abundunt mucin production and its outcome.