Mucinous carcinoma of the breast is a rare histological type, which represents between 1 and 4% of breast cancers. Treatment does not differ from other histological types, and it occurs more frequently in older adult women. Prognosis is good. We report the case of a 72-year-old patient with a 1-year disease course characterized by the appearance of a slow-growing tumor in the left upper quadrant of the left breast, in which the core biopsy showed mucinous breast carcinoma of a low nuclear grade. The patient underwent quadrantectomy plus a sentinel node biopsy, which confirmed the initial diagnosis.