BACKGROUND Spontaneous regression of a malignant tumor is defined as "the partial or complete disappearance of a malignant tumor in the absence of any treatment." Herein, we report a case of occult breast cancer with axillary lymph node metastasis that showed spontaneous tumor regression based on the histopathological findings. METHODS A 67-year-old woman presented with left armpit pain and a lump. Previous examination by another doctor revealed swelling of the left axillary lymph node, but it was difficult to identify the primary lesion. Needle biopsy of the left axillary lymph node revealed malignant tumor tissue with extensive necrosis on histological examination. On initial examination at our hospital, the left axillary lymph node was observed to have shrunk compared to previous observations. Moreover, findings indicated a suspected concentrated cyst in the left breast, with slight contrast enhancement on magnetic resonance imaging. Considering a diagnosis of occult breast cancer with axillary lymph node metastasis, excisional biopsy was performed for the left breast mass and axillary lymph node dissection for left axillary lymph node metastasis. Histological examination revealed a micro adenocarcinoma with lymphocyte infiltration in the left breast, and the viable tumor in the left axillary lymph node had disappeared. The histopathological findings of the primary tumor and dissected lymph nodes suggested the possibility of spontaneous regression of both the primary and metastatic lesions, because effective preoperative therapy was not performed.