Extranodal follicular dendritic cell sarcoma (FDCS) is an extremely uncommon tumor with only a single case arising in the breast having been reported. We describe the clinico-pathological features of an additional FDCS of the lower outer quadrant of the right breast in a 40-year-old woman. The tumor showed three patterns of growth, i.e., diffuse, myxoid and fascicular. The neoplastic cells were large, polygonal, with a slightly eosinophilic cytoplasm and oval or convoluted nuclei. They were intermingled with small lymphocytes, plasma cells and a few bizarre multinucleated giant cells. In the fascicular areas, the cells were spindled, while in the myxoid areas they showed a dendritic-like appearance, with long cytoplasmic processes. Mitoses were numerous and often atypical. The neoplastic cells were intensely immunoreactive for CD21, S-100 protein and epithelial membrane antigen, and focally for CD35, CD68 and cytokeratins. Polymerase chain reaction analysis did not reveal any Epstein Barr virus genome in the neoplastic tissue. Electron microscopy highlighted numerous interdigitating cytoplasmic processes with intercellular junctions of the serrated, immature desmosomal or undifferentiated types. The post-surgical course of the patient was uneventful and she is currently free of disease 19 months after surgery.