Occult breast cancer presenting with axillary lymph node metastases is uncommon, and inflammatory breast cancer (IBC), as a subtype, is quite rare. Here we describe a case of IBC, which arose as an unknown primary carcinoma; the patient presented with axillary lymph node metastasis, and was successfully treated with trastuzumab and vinorelbine. Specifically, a 55-year-old woman presented with right axillary lymphadenopathy. Although she underwent various examinations, the primary site of the disease was not revealed. Axillary lymph node dissection was performed, and the lesion was diagnosed as a poorly differentiated adenocarcinoma. The patient chose to be treated by alternative medicine. About 6 months later, she was referred to our hospital, due to marked bilateral neck and axillary lymph node swelling. She presented with diffuse right breast enlargement, redness, and peau d'orange. Computed tomography (CT) of the breast showed skin thickening and swelling of the right breast.F-18 Fluorodeoxyglucose positron emission tomography (FDG-PET) showed FDG uptake in the right breast. The patient was clinically diagnosed with IBC. Because overexpression of the human epidermal growth factor receptor 2 (HER2) was found in the specimen from her right axillary lymph node, she was treated with trastuzumab and vinorelbine. Two months after the start of chemotherapy, CT revealed a complete response in the lymph nodes, and the skin thickening and parenchymal edema of the right breast had improved. FDG-PET was also performed at this time, and revealed no FDG uptake in either the right breast or the lymph nodes.