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Background: The colorectal tumors limited within the mucosa or superficial submucosa(sm-small) can be completely treated endoscopically since the probability of lymph node metastasis is almost nil. Histologically, the depth of submucosal tumor invasion was regulated by equally divided three levels of the submucosa in the vertical direction. Tumors of sm-small were defined as those limited within the superficial 1/3 of the submucosa, tumors of sm-massive, extending from the middle 1/3 of the submucosal layer to the deeper portion near the muscularis propria. Therefore, pretherapeutic diagnosis whether the depth of tumor invasion is limited within sm-small or deeper than sm-massive is extremely important for the treatment strategy. Aims: To evaluate the usefulness of EUS in the diagnosis of depth of colorectal cancer invasion, and compare the ability of EUS with conventional colonoscopy in the diagnosis of submucosal cancer invasion. Methods: Between May 1985 and September 1999, EUS was performed in 563 patients with colorectal cancer. All of the tumors were resected and confirmed histologically. Especially, preoperative EUS diagnosis of submucosal cancer staging was investigated in comparison with conventional colonoscopy in 77 patients with colorectal submucosal cancer. EUS criteria for the depth of submucosal cancer invasion were defined as SM-small and SMmassive according to the deepest portion of the hypoechoic tumorous mass compared with the third hyperechoic layer in the vertical direction. Colonoscopic criteria for the SM-massive were based on the following findings; marginal submucosal elevation, erosion or ulceration on the tumor, marked elevation on the tumor and fold convergence. Results: EUS correctly diagnosed pTis in 98 of 118 patients, pT1 in 62 of 80 patients, pT2 in 42 of 84 patients, pT3 in 248 in 276, and pT4 in 5 of 5 patients. The overall accuracy rate of EUS was 81%(455/563) according to TNM classification. Pretherapeutic diagnosis was investigated whether the depth of tumor invasion was sm-small or sm-massive. The accuracy rates of EUS and colonoscopy were 23/31(74%) and 19/31(61%), respectively, in sm-small, and 41/46(89%) and 38/46(83%), respectively, in sm-massive. The overall accuracy rates with EUS and colonoscopy were 83%(64/77) and 74%(57/77), respectively. Conclusion: EUS is a useful diagnostic method for determining the depth of colorectal cancer invasion according to TNM classification and subclassification of submucosal cancer.