Intestinal endometriosis of the rectum and sigmoid colon, occurring in up to 34% of pelvic endometriosis, mimics a wide number of conditions that are difficult to differentiate from inflammatory or malignant diseases. Herein we report the first case of transmural endometriosis concomitant with advanced primary rectal adenocarcinoma, presenting with obstructive symptoms. Correct diagnosis based on morphological identification and immunohistochemical characterization of the two entities is crucial for treatment.