A case of adenomyoepithelioma of the breast in a 58-year-old woman is described. The tumor was a solitary, solid mass measuring 35 x 30 mm. Histologically, the tumor was composed of a proliferation of both epithelial and myoepithelial cell components. The epithelial cell component showed extensive columnar cell change with clear cytoplasm and focal exaggerated proliferation with cytologic atypia and increased mitotic figures. DNA ploidy analysis revealed aneuploidy. DNA ploidy analysis might be used to predict the biological behavior of adenomyoepithelioma.