We report a case of a 66-year-old woman who presented with multiple painless masses in both breasts. Prior bilateral biopsies were diagnosed as Rosai-Dorfman disease (Sinus Histiocytosis with Massive Lymphadenopathy). A recent lumpectomy specimen revealed a gray-white smooth cut surface with a discrete masslike lesion. The histopathology demonstrated a fibrotic breast parenchyma with foci of dense fibrosis and scattered inconspicuous breast epithelium surrounded by lymphocytes that formed aggregates and follicles with germinal centers. The inflammation was in a periductal, perilobular, and perivascular distribution. In addition, an exuberant inflammatory response with histiocytes and fibroblasts was present. This inflammatory response focally surrounded areas of fat necrosis and formed noncaseating granulomas with rare multinucleated giant cells. This process had infiltrative, ill-defined edges and involved the subcutaneous tissues. The overlying epidermis was normal. The final diagnosis was diabetic mastopathy with an exuberant lymphohistiocytic response. The differential diagnosis included Rosai-Dorfman disease, inflammatory myofibroblastic tumor, granulomatous mastitis, sclerosing lipogranulomatous response/sclerosing lipogranuloma, lupus panniculitis, and rheumatoid nodules. Immunohistochemical studies and flow cytometry confirmed the polyclonal nature of the lymphoid infiltrate. After the histologic evaluation, we inquired if the patient had a history of diabetes mellitus, and learned that she did have type 2 noninsulin-dependent diabetes mellitus. In conclusion, we report a case of diabetic mastopathy that presents with bilateral tumorlike masses and an unusual exuberant lymphohistiocytic response with granuloma formation. The pathologist may not be provided with a history of diabetes mellitus, but the characteristic fibrosis, lymphocytic ductitis/lobulitis, and sclerosing lobulitis with perilobular and perivascular lymphocytic infiltrates should provide clues for an accurate diagnosis, even when an exuberant and an unusual lymphohistiocytic response is present. A timely accurate diagnosis can help limit repeat surgeries in this vulnerable group of patients.