Three unusual morphological polymorphous adenocarcinomas (PACs) were collected from our files, 1 case with high-grade transformation, 1 case with solid sheets and nested architectures, 1 located at parotid gland. Our emphases are high-grade transformation and its histopathological, immunohistochemical, possible mechanism of tumorigenesis, and differential diagnoses. Microscopically keeping with the uniformity of the cytological morphology at a higher atypical nucleus and showed mitotic activity, the high-grade area showed solid nested and cribriform patterns; the nuclei were enlarged and palely stained predominantly, focal giant tumor cells could be seen, and immunohistochemistry suggested that both ductal and myoepithelial differentiation were present. PAC is an infiltrating tumor with cytological uniformity and architectural diversity, high-grade transformation is very rare, which may indicate a more aggressive behavior and should be differentiated from other mimics.