We report the case ofan elderly patient with squamous cell carcinoma(SCC)ofthe breast, which showed negative conversion after endocrine therapy. An 82-year-old woman with a diagnosis ofER -positive SCC(cT1N0M0, StageⅠ)received primary endocrine therapy with 5 hormonal medicines. Following the endocrine therapy, she underwent mastectomy and axillary node resection. Histological examination confirmed a diagnosis of ER-negative SCC. At 10 months after the operation, she was doing well with no evidence ofdisease without postoperative adjuvant therapy. Thus, clinicians should be aware that the ER status may change after primary endocrine therapy. For elderly patients with breast cancer, it is important to be aware that primary endocrine therapy can become ineffective due to ER-negative conversion and aging due to prolonged treatment.