A 43-year-old woman presented to our department with left breast lump, measuring 10 mm x 10mm x 6mm. On physical examination and mammography, the lump was suspected to be due to mastopathy, but malignancy could not be excluded based on ultrasonography. Excisional biopsy showed that it was an invasive lobular carcinoma. A modified radical mastectomy was performed after obtaining informed consent. She remained disease-free seven years after surgery.