Background: The development of cancer from adenomyotic foci is a rare occurrence. The diagnosis is frequently delayed because of the absence of tumor in the eutopic endometrium. Case Report: We present a case of a 64-year-old postmenopausal woman with irregular vaginal bleeding and dull abdominal pain. Hysteroscopy was negative and hormonal treatment was continued. Nine months later, persisting symptoms necessitated endometrial biopsy revealing an atrophic endometrium. Hydrosonography suggested an endometrial polyp of 14 x 7 mm with a surrounding regular thin endometrium and a diffusely inhomogeneous ultrasonographic pattern throughout the myometrium. Hysteroscopic excision of the endometrial polyp was performed. Biopsies obtained during operative hysteroscopy showed a well differentiated endometrioid endometrial carcinoma. A laparoscopically assisted vaginal hysterectomy with bilateral salpingo-oophorectomy, pelvic lymphadenectomy and peritoneal cytology was performed. Pathologic examination revealed an atrophic endometrium and a Stage IB (FIGO 2009) well differentiated endometrioid endometrial carcinoma with prominent squamous differentiation originating from nodular adenomyosis. This ectopic localization of the endometrioid carcinoma added to a diagnostic delay of 12 months. Conclusion: Endometrial cancer arising from uterine adenomyosis may be difficult to diagnose. Awareness of this entity and careful ultrasonography are likely to reduce diagnostic delay.