A 48-year-old woman was diagnosed with left-sided breast cancer. The initial operation entailed left mastectomy, sentinel lymph node biopsy, and tissue expander (TE) insertion. On postoperative day 1 (POD1), bloody discharge from the chest drain was observed and it continuously reduced under compression. However, between POD13 and POD14, the amount of discharge increased (Fig. 1). A computed tomography scan of the chest revealed a hematoma underneath the left pectoralis major muscle (Fig. 2). The next day, POD14, 250 mL of hematoma was removed. No further bleeding occurred, and the patient was discharged on POD20 (Fig. 3).Ten months later, breast reconstruction was performed using a deep inferior epigastric perforator (DIEP) flap.