It can be extremely difficult in some cases to distinguish atypical polypoid adenomyomas (APAs) from invasive adenocarcinoma in an endometrial curettage or biopsy specimen. In order to determine if immunophenotypic features can be exploited to differentiate between these two entities in problematic cases, a series of APAs and myoinvasive well-differentiated endometrial carcinomas (WDCAs) were studied with a panel of standard immunohistochemical markers. All 23 APAs had stromal smooth muscle actin (SMA) reactivity, 12 of 23 had variable degrees of stromal desmin reactivity, and nine of 22 had CD34-positive stromal cells. All epithelial components of the APAs were cytokeratin (AE1 and CAM5.2) positive, whereas 22 of 23 were positive for estrogen receptor (ER) and progesterone receptor (PR). Among the 10 myoinvasive WDCAs, all contained at least some SMA-positive stromal cells, seven of 10 desmin-positive stromal cells, and four of eight CD34-positive stromal cells. All carcinomas studied demonstrated CAM5.2 and PR-positive epithelia; nine of 10 were ER positive. We conclude that the immunophenotype of APAs does not differ significantly from well-differentiated endometrial adenocarcinoma and that immunophenotyping is of little value in distinguishing APA from carcinoma. Because the stroma in APAs histologically and immunophenotypically more closely resembles a hybrid myofibromatous stroma, we prefer to refer to these lesions with the modified designation "atypical polypoid adenomyofibroma," although "APA" may be retained for clinical use.