Diabetic mastopathy is a clinicopathologic entity which was first described as a dense fibrous breast mass in insulin-dependent diabetes mellitus. The purpose of this article was to document diabetic mastopathy histologically which had been diagnosed as fibrocystic disease and to avoid unnecessary surgical procedures in breast mass simulating malignancy in diabetic patients. We examined eight excisional breast biopsies from seven patients. Three diabetic patients with type I insulin-dependent diabetes mellitus presented with bilateral (2 patients) or unilateral (1 patient) rapidly growing palpable breast masses. Four patients with type II noninsulin-dependent diabetes mellitus presented with bilateral (2 patients) or unilateral breast mass (1 patient). One patient had no symptoms. All of them had late complications of diabetes mellitus such as nephropathy, neuropathy and retinopathy. Mammographic findings such as ill- defined mass density and asymmetric increased density suggested malignancy. However, all of them had been diagnosed as fibrocystic disease. On review, the most consistent pathologic finding was keloid-like stromal fibrosis. Others were ductitis or ductulitis, thickening of basement membrane of ducts or ductules, mononuclear perivasculitis and lobulitis. Six of eight breast satisfied all five criteria for diabetic mastopathy.