A 61-year-old woman was found to have calcifications in the CD region of the left breast. She had previously undergone total hysterectomy and bilateral oophorectomy for endometriosis at the age of 37 years. Since age 59 years, she had been attending an otorhinolaryngology clinic because of vertigo. Blood tests showed no abnormal findings. Left breast cancer (cT1N0M0, stage Ⅰ)was diagnosed, and left mastectomy and sentinel lymph node biopsy were performed. She developed postoperative nausea, and at 37 hours postoperatively, she was unable to communicate and exhibited suspected delirium. At 43 hours postoperatively a tonic-clonic seizure occurred. Hyponatremia, with serum sodium of 114 mEq/L, was present. Sodium supplementation was provided, and the patient became capable of communication 8 hours after seizure onset(Na 121 mEq/L). A hyponatremic tonic-clonic seizure is extremely rare after breast cancer surgery, and the abnormal behavior of the present patient 31 hours after surgery was also highly unusual. With such an unusual presentation, the possibility that something specific is happening must be considered. This case gave us the opportunity to review patient management after breast cancer surgery, emergency response and preparations, and nursing education from the medical safety perspective.