We treated a patient with breast cancer associated with McCune-Albright syndrome. A 40-year-old woman with a history of precocious puberty visited our hospital complaining of a mass in the upper lateral quadrant of the right breast. Although bone scintigraphy revealed multiple high uptake of 99mTc, plain X-ray demonstrated ground-glass appearance, suggesting fibrous dysplasia rather than bone métastases. Serum levels of tumor markers and alkaline phosphatase were within the normal range. Breast cancer associated with McCune-Albright syndrome was diagnosed, and she subsequently underwent breast conserving surgery, excision of abdominal wall myxoma and bone biopsy of the left clavicula. The bone lesion was histologically confirmed as fibrous dysplasia. Although McCune-Albright syndrome is a rare clinical entity, it should be considered as a possible differential diagnosis of bone metastasis in patients with breast cancer. As recent molecular studies have suggested genetic mutations in McCune-Albright syndrome, this syndrome may possibly predispose patients to breast cancer.