Breast neoplasm may develop in ectopically located glandular tissue. This paper presents an interesting and rare case of a 50-year-old female who despite regular mammography screening examination developed an invasive accessory breast cancer. Clinical examination revealed a 2 cm - tumour localized 4 cm below the left inframammary fold. The lesion was immobile, the skin and the atrophic nipple were retracted, the tumour infiltrated the thoracic wall. Oligobiopsy and additional examinations showed an invasive stage IIIB ductal breast cancer (Bloom II, G-2). The receptor status was: ER(+), PGR(+), HER2(-). The increased level of cancer antigen 15.3 was found. The patient was submitted to pre-operative chemotherapy. She also underwent surgery and subsequently post-operative chemotherapy and radiotherapy. On the basis of the presented case, it could be concluded that the accessory mammary glands are out of the image of screening breast examinations. Accessory breast cancer is usually diagnosed by clinical examination and ultrasonography. Preventive resection of accessory breast in women at high risk of developing breast cancer can be considered as the treatment of choice in most patients.