A rare case of invasive ductal carcinoma within a fibroadenoma of the breast in a 42-year-old woman is reported. The patient had a well-defined mass, measuring 6.0 x 5.0 cm, in the upper lateral quadrant of the left breast. Physical examination suggested fibroadenoma. Ultrasonography and mammography revealed some malignant findings. Needle biopsy demonstrated fibroadenoma. Frozen section revealed invasive ductal carcinoma, scirrhous type, arising in a fibroadenoma; muscle-preserving mastectomy was performed. Only 16 cases of carcinoma within a fibroadenoma of the breast have been reported in the literature in Japan. Carcinoma in a fibroadenoma should be treated on the basis of the therapeutic criteria for ordinary carcinoma.