The cytologic features of a fine needle aspiration biopsy of a malignant intraductal myoepithelioma of the breast are described. On cytology the tumor cells were shed in cohesive groups consisting of ill-defined polygonal and spindle cells. The latter, which had centrally located, cigar-shaped nuclei, showed a fascicular pattern. Despite cellular multilayering, there was a halo-like transparency around the nuclei, suggesting that many cells had clear or pale cytoplasm. Mild nuclear atypia was occasionally present. Mitotic figures were also observed. With immunostaining, clustered cells showed a diffuse positive reaction for alpha smooth muscle actin (alpha-SM-actin). The tumor cells proliferated intraductally, as in a conventional intraductal carcinoma with a comedo or solid pattern. Characteristically, zones of clear, polygonal cells were situated at the ductal periphery. Toward the center of each duct, tumor cells were transformed into nonclear cells, and some were further transformed into spindle cells that tended to form fascicles. Immunohistochemically, most of the tumor cells expressed alpha-SM-actin.