Atypical polypoid adenomyoma (APA) is a rare polypoid tumor of the uterus composed of atypical endometrial glands and smooth muscle cells. Concomitant development of endometrial adenocarcinoma in APA remains infrequent. We report a case of the coexistence of endometrioid adenocarcinoma in APA. A 41-year-old patient presented with abnormal genital bleeding. A polypoid mass was extruded from the external cervical os. She underwent transcervical resection of the polypoid mass arising from the

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... uterine segment. Pathological examination revealed APA with the foci of well-differentiated endometrioid adenocarcinoma. Subsequently, she underwent total hysterectomy and bilateral salpingo-oophorectomy. No residual malignant lesions were found. Awareness of the close association of APA with the development of endometrial cancer is warranted. A meticulous pathological evaluation of specimen of APA is necessary for the detection of the coexistence of endometrial cancer.