A surgically resected case of giant malignant phyllodes tumor of the breast associated with a hypoglycemic attack is reported. A 54-year-old woman was referred to our hospital with loss of consciousness and a huge chest wall tumor. She was diagnosed as having a malignant phyllodes tumor by core needle biopsy and underwent palliative simple mastectomy because lung metastasis was detected on computed tomography and by other imaging modalities on admission. The preoperative laboratory data revealed a very low fasting blood sugar level of 37 mg/dl. After removal of the tumor, the blood sugar level gradually normalized (90-100 mg/dl) and the plasma insulin-like growth factor-II (IGF-II) level promptly decreased. The IGF-II level of tumor extracts was high (2500 ng/g wet weight) and the majority of atypical cells stained positively for IGF-II immunohistochemically. These findings suggested that the patient's hypoglycemia was associated with IGF-II produced by a giant malignant phyllodes tumor that consumed glucose.