Two cases of adenomyoepithelioma of the breast with malignant transformation by monophasic population of cells are presented. The underlying benign adenomyoepithelioma with typical biphasic architectural pattern was identified and represented at least 30% of the tumor in each case. In both cases, malignant portion of tumor was composed of relatively uniform monophasic population of highly atypical cells. The malignant component in case 1 was positive for pan cytokeratin, myoepithelial markers, and basal-type cytokeratins and also focally positive for luminal-type of cytokeratins, but negative for hormone receptors (estrogen and progesterone) and HER-2/neu protein overexpression. The malignant component in case 2 was positive for spectrum of myoepithelial markers but negative for luminal cytokeratins, hormone receptors and HER-2/neu protein overexpression. The bilinear immunophenotype in the case 1 suggests that the malignant tumor may have developed from precursor multipotent cells that can differentiate into both luminal epithelial and myoepithelial cells, although malignant component in case 2 appears to be the of pure myoepithelial phenotype.