Endoscopic ultrasonography was pre-operatively performed in 164 patients with colorectal cancer, and the resected specimens confirmed to be cancerous histologically. Normal colorectal wall was visualized by endoscopic ultrasonography as a five-layered structure. The first, third, and fifth layers were hyperechoic, and the second and fourth layers were hypoechoic. The first and second layers comprised the mucosa, the third layer the submucosa, the fourth layer the muscularis propria, and the fifth layer the subserosa and serosa (adventitia). By this technique, cancer of the colon appeared as a hypoechoic mass with an intermediate echo level between the third hyperechoic and fourth hypoechoic layers. Endoscopic ultrasonographic determination of the depth of tumor invasion was based on abnormal changes of these layered structures and adjacent organs. Overall, the accuracy rate of endoscopic ultrasonography in the diagnosis of the depth of colorectal cancer was 83%. A hypoechoic round mass adjacent to the cancer was interpreted as a metastatic lymph node. The sensitivity and specificity of endoscopic ultrasonography in the diagnosis of lymph node metastasis were 68% and 70%, respectively. The overall accuracy rate of tumor staging by endoscopic ultrasonography according to Dukes' classification was 62%. Endoscopic ultrasonography is a valuable tool in the management of colorectal cancer. It has a high accuracy rate in determining the depth of tumor invasion. The pre-operative information obtained with this tool may influence the choice of therapy.