To confirm the origin of cancer found in both the endometrium and the myometrium is difficult. Cancer may spread from the endometrium into adenomyotic foci or vice versa. Also, premalignant changes may arise at either or both sites. Investigating disease origin enhances our understanding of pathophysiology and prognosis. Additional critical questions are whether women with adenomyosis have a higher risk of endometrial cancer; whether the invasive properties and prognosis of cancer in adenomyosis differ from those arising in the eutopic endometrium and whether the ectopic glandular tissue in adenomyosis becomes altered in the presence of eutopic endometrial cancer. A final question is whether cancer arising within adenomyosis carries a worse prognosis because of its location within the myometrium and the possibility that the presence of adenomyosis facilitates invasion of cancer arising in the eutopic endometrium. The present review explores currently available literature in an attempt to answer these questions and to examine clinical presentations, diagnostic criteria, pathogenesis and prognosis.