We present a case of a 56-year-old female, with a familial history of breast, lung and brain cancer, which revealed a breast tumor, located in the upper outer quadrant of the left breast. During the routinely staging examinations, a 15 cm intrathoracic tumor was found in the upper left mediastinum, penetrating the pericardium and a smaller tumor, in the left side of parietal pleura. Core biopsies from both lesions, revealed a lobular carcinoma of the breast classic type, grade II (e-cadherin-, ER+, PR+, Her-2-, Ki-67 10%) and a B3 thymoma (CK19+, CD5+) penetrating the pericardium and the left lung. A synchronous removal of both tumors was scheduled, including median sternotomy and left intrapericardial pneumonectomy, followed by a modified radical left mastectomy and a sentinel lymph node biopsy. The postoperative course was uneventful. This case advocates that thymoma patients appear to have a predisposition towards developing additional neoplasms, as breast carcinoma. Clinicians should be aware of the increased incidence of extrathymic cancers, occurring in thymoma patients.