This report describes the histologic and immunohistologic features of an intraductal myoepithelial tumor that developed in the breast of a 61-year-old woman. Histologically, the tumor proliferated intraductally, with both a comedo or doughnut pattern and a solid pattern containing narrow fibrovascular cores, mimicking what appeared to be a conventional intraductal carcinoma. No fine papillary or arborizing growth or cribriform formation was observed. Tumor cells at the ductal peripheral zone were polygonal and clear with abundant glycogen in the cytoplasm; they were transformed into nonclear cells with slightly eosinophilic cytoplasm toward the center of the involved ducts. Occasionally, nonclear cells were elongated, with a centrally located cigar-shaped nucleus. These elongated or spindle cells tended to show a fascicular and streaming pattern similar to that of a smooth muscle tumor. Immunohistochemically, alpha smooth muscle actin (alpha-SM-actin) and S-100 protein were expressed in most of the nonclear cells. While clear cells also had a positive reaction for S-100 protein, they were mostly negative or barely positive for alpha-SM-actin. Epithelial membrane antigen (EMA) was also positive in a certain number of polygonal cells. These results support the myoepithelial nature of the present tumor, and some cells might also be immunologically differentiated into ductal epithelial cells. In addition to cytological atypia, frequent mitoses, and central necrosis within ducts, there was a minimal but evident stromal invasion, suggesting histological malignancy in this peculiar tumor.