BackgroundEndometrioid adenocarcinoma is usually diagnosed by endometrial curettage with a positive rate of 94%, while a hysteroscopic examination can increase the positive rate. Differently, endometrioid adenocarcinoma arising from adenomyosis (EAAFA) is always misdiagnosed, even after endometrial curettage or hysteroscopy. EAAFA is rarely reported.Case descriptionWe reported two cases of EAAFA with long-term follow-ups of 9–10 years. The two cases were misdiagnosed even if endometrial curettage or hysteroscopy was performed. One case complained of postmenopausal vaginal bleeding with elevated CA199, and she was diagnosed by fast-frozen pathology during hysterectomy, followed by bilateral salpingo-oophorectomy and pelvic lymphadenectomy. In another case, a premenopausal woman with acquired progressive dysmenorrhea was diagnosed as EAAFA by the histopathological result after a transvaginal hysterectomy and had to undergo reoperation. The FIGO stage was IB in both cases. Chemotherapy or radiotherapy was performed after the operation. Patients were followed up for 9–10 years, with no metastasis or recurrence being observed.ConclusionThe diagnosis of EAAFA is always delayed because of tumor-free eutopic endometrium and negative results of the endometrial curettage or hysteroscopy examination. Fast-frozen pathology of the whole uterus helps diagnose EAAFA precisely and avoids reoperation. Adenocarcinoma foci of EAAFA usually involve the myometrium deeply. A better prognosis of EAAFA should be expected due to good differentiation and negative lymphovascular space invasion.