Malignant changes in endometriosis are often reported, but those in adenomyosis are rare. We report a case of endometrioid adenocarcinoma arising from adenomyosis. Case Presentation. A 57-year-old woman presenting with vaginal bleeding was referred to our hospital. Cytological tests of endometrium revealed atypical glandular cells. Fractional endometrial curettage revealed normal endometrium without atypia. Magnetic resonance imaging (MRI) revealed multiple myomas. The endometrium was slightly enhanced on T1-weighted imaging and endometrial cancer was suspected. Myometrial invasion was not evident. The patient was admitted and semiradical hysterectomy with bilateral salpingo-oophorectomy and pelvic lymphadenectomy was performed. Histopathological study revealed grade 1 endometrioid adenocarcinoma. Although the lesion was located in the muscle layer of the corpus and invaded more than half of it, the endometrium was intact. Pelvic lymph node metastasis was noticed. No cervical invasion or metastasis to the adnexa was seen. We diagnosed the case with a stage 1B endometrioid adenocarcinoma originating from adenomyosis. Adjuvant chemotherapy was then performed in the form of 5 cycles of paclitaxel (180 mg/m(2)) and carboplatin (AUC = 5). Five years later, right lung metastasis and right para-aortic and pelvic lymph nodes metastasis were noticed. Paclitaxel and carboplatin are now being administered.