Diabetic mastopathy is an uncommon tumor-like proliferation of fibrous tissue of the breast that usually occurs in a patient who has suffered from type I diabetes mellitus of long duration. Here we report a rare case of diabetic mastopathy that occurred in type II non-insulin dependent diabetes mellitus. This patient was a 63-year-old postmenopausal woman. Mammography, ultrasonography and MR imaging could not distinguish it from breast cancer. Although the core needle biopsy specimen showed fibrosis without evidence of malignancy, excisional biopsy was performed. Histological findings demonstrated typical diabetic mastopathy with keloid-like fibrosis, perivascular lymphocytic infiltration, and lymphocytic lobulitis without evidence of malignancy. These lymphocytes were composed predominantly of B-cells. Five months after surgical biopsy, a nodular formation approximately 4 cm in diameter recurred adjacent to the resected end of the biopsy.