A 50-year-old woman presented with a mass in the upper outer quadrant of her left breast, which proved to be a mucinous cystadenocarcinoma by biopsy. An F-FDG PET/CT was performed for staging, which showed that the breast tumor was the only lesion with abnormal F-FDG avidity. Breast-conserving surgery was performed following a negative sentinel lymph node biopsy of the left axilla, and primary mucinous cystadenocarcinoma of the breast was finally diagnosed.