We report a patient with a massive mucinous carcinoma of the breast that had been untreated for 6 years. A 47-year-old premenopausal woman presented with a large right-breast mass. Although she had noticed a lump in the right breast 6 years previously, she had not sought treatment. The tumor had enlarged gradually and become ulcerated. With a diagnosis of advanced breast cancer with skin invasion, she underwent right mastectomy with a free skin graft. No lymph node metastases or distant metastases were detected. The histologic diagnosis of the tumor was pure mucinous carcinoma of the breast with no component of ordinary invasive ductal cancer. She has been well without evidence of tumor recurrence for 7 years after the surgery. The prognosis for mucinous carcinoma of the breast has been recognized as relatively good. The results in our patient are consistent with the biological behavior of this carcinoma.