Two polypoid submucosal uterine lesions were examined histologically and immunohistochemically with monoclonal antibodies to desmin and alpha smooth muscle actin. One case comprised a leiomyoma and the other a polypoid form of adenomyosis. Both polyps had prolapsed through the external cervical os. The lesions had an ulcerated surface with focal areas of marked increased cellularity and pronounced vascularity throughout, such that they mimicked a low grade endometrial stromal sarcoma infiltrating the myometrium. The cellular areas showed diffuse positivity for desmin and alpha smooth muscle actin, confirming them to be of smooth muscle origin. The changes of marked hypercellularity and pronounced vascularity within polypoid submucosal uterine lesions have not been emphasised in published reports up to now. Pathologists should be aware of these morphological features in order to avoid misdiagnosis of such cases as endometrial stromal sarcomas. The changes described here are likely to be secondary to trauma associated with a polypoid lesion prolapsing through the external cervical os.