There are several case reports of adenocarcinomas developing within adenomyosis. However, there is no report demonstrating the natural course from adenomyosis to adenocarcinoma. We report a patient (a 41-year-old Japanese woman) who was observed every 6 months after being diagnosed with adenomyosis at our University Hospital. Although she went through menopause at age 51, she occasionally complained subsequently of abnormal genital bleeding. Eleven years after the initial diagnosis, endometrial cytology revealed the presence of malignant cells. Pelvic magnetic resonance imaging (MRI) demonstrated replacement of the adenomyotic lesion by a poorly demarcated lesion, compared to the findings on prior MRI. Consequently, we performed a modified radical hysterectomy and pelvic lymph node dissection, under a presumptive diagnosis of adenocarcinoma arising in adenomyosis. Histological diagnosis revealed an endometrioid adenocarcinoma (G3) transformed from adenomyotic epithelium, which was classified, according to the International Federation of Gynecology and Obstetrics, as stage Ic, pT1cN0M0. In this patient, periodic MRI evaluations, in conjunction with pathological examination, identified the transformation from adenomyosis to adenocarcinoma.