A rare kind of a breast tumor was identified in a woman of 47. The disease ran a malignant course and presented difficulties for morphological verification. Histological appearance showed areas of spindle-shaped or lamellar cells occasionally encircled with homogeneous hyalin-like substance. Locally, there were structures of alveolar or glandular type with an irregular basal membrane and loose position of cells. The primary histological diagnosis suggested a tumor of mesenchymal origin. Followed by immunomorphologic (monoclonal antibody to KL 1) and electron-microscopic examinations, the diagnosis was verified providing evidence for epithelial nature of the tumor cells, loose, forming glandular-like structures and simulating stromal elements with ultrastructural features typical of myoepithelial differentiation. The evidence obtained justify the tumor definition as a rare neoplasm of the breast known in the literature as myoepithelioma.