A case of endometrioid adenocarcinoma supposedly arising from endometriosis of the rectum is reported. Malignant transformation is uncommon but a well-known complication of endometriosis. In the present case, it was proved by histopathological findings and immunophenotype such as cytokeratin7+/cytokeratin20-/estrogen receptor+. The cause of rectal endometriosis in this case might have been related with previously received hormone replacement therapy for ovarian endometriosis. Following surgical removal of the lesion, this patient underwent adjuvant chemotherapy with paclitaxel and carboplatin, although this kind of therapy is still controversial as to its effectiveness.