Compared with female breast cancer, male breast cancer (MBC) has an extremely low morbidity, later staging and fewer breast tissues. The lumps are easier to invade in the center and the majority of the cases are positive for metastatic lymph node, with the typical clinical manifestation as a painless mass in partial breast. MBC with an unknown primary tumor is rare and is often prone to misdiagnosis, resulting in a delay in correct treatment. Such a case is extremely significant for clinical reference. The current study presents a 58-year-old male who developed a painless mass in the left armpit and received armpit mass biopsy and pathological examination which showed glandular cancer, with a high possibility of mammary primary tumor. The patient was administered four cycles of paclitaxel plus oxaliplatin chemotherapy. However, three months later, the patient identified novel disseminated lymph nodes in the left armpit. The initial pathological section and paraffin blocks were re‑examined and the patient was finally diagnosed with breast invasive ductal carcinoma based on the metastases pathology and immunohistochemical examination. No breast mass was found on physical examination of the patient and the tumor markers, including cancer antigen 125 and carcinoembryonic antigen, were normal. No primary tumors were observed in the mammography and PET-CT and the primary tumor was not found following the left breast modified radical mastectomy.

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