Endoscopic submucosal dissection (ESD) is a state-of-the-art method that enables resection of larger tumors than those resectable by conventional endoscopic mucosal resection (EMR). However, the individual role of each method in the treatment of colorectal tumors remains undetermined.To consider the respective indications of ESD and EMR for colorectal tumors, we analyzed the results of the two treatments retrospectively.Tumors treated by ESD (44 tumors) were significantly larger, more often located in the rectum and more often coexistent with cancer than those treated by EMR (512 tumors). EMR was used in the majority of adenomas, and showed high rates of both one-piece resection (OPR) and complete resection (CR) for adenomas less than 20 mm. However, for adenomas and cancers greater or equal to 20 mm, the CR rate for EMR was significantly lower than that for ESD because of the incidence of OPR with a positive lateral margin (16% vs 0% with ESD vs EMR). Histopathology (cancer), size (> or =20 mm) and macroscopic type (laterally spreading tumors) were shown to be significant risk factors for that incidence. For tumors with these factors, ESD showed a higher CR rate than did EMR. However, ESD required longer operating times and tended to have a higher rate of perforation compared with EMR. ESD was aborted halfway in seven cases due to technical difficulties and perforation.ESD and EMR have different characteristics as treatment for colorectal tumors. Careful evaluation of the lesion and of the balance between benefits and risks are mandatory before selecting either of these treatments for colorectal tumors.