The patient was a 57-year-old woman with Castleman's disease. The follow-up CT scans obtained during the treatment of Castleman's disease, detected a 15mm nodule in the right breast AC area, and the patient was diagnosed with breast cancer. Lymphadenopathy was noted on both sides of the axilla; however, it was considered to be due to Catsleman's disease. Mastectomy and sentinel lymph node biopsy were performed with preoperative diagnosis of cT1cN0M0, cStage Ⅰ. Rapid diagnosis of the sentinel lymph node during the operation showed a metastatic tumor measuring 3mm and axillary dissection was performed. However, no metastasis was found in the dissected lymph node, which was, therefore, considered as an enlargement due to Castleman's disease.