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In rectal cancer, depth of infiltration and metastatic involvement of lymph nodes are important prognostic factors. The correct choice of operative treatment depends on the extent of the disease. In a prospective study, the value of endorectal ultrasound in staging rectal cancer was evaluated, and factors affecting the method's accuracy are discussed. The overall accuracy in staging depth of infiltration was 89 percent. Overstaging occurred in 10.2 percent, understaging in 0.8 percent. Tumors of the lower rectum are incorrectly staged in 16.7 percent, whereas tumors of the middle and upper rectum had an incorrect staging in 6.3 percent (P<0.001). Compared with computed tomography, endorectal sonography is the more accurate staging method (74.7vs. 90.8 percent). In staging lymph nodes, the overall accuracy was 80.2 percent, sensitivity was 89.4 percent, specificity was 73.4 percent, positive predictive value (PPV) was 71.2 percent, and negative predictive value (NPV) was 90.4 percent. The staging accuracy depends on the size of the node. Endorectal ultrasound is a safe, inexpensive, and accurate staging method, in the assessment of both depth of infiltration and nodal status. The results are strongly related to the experience of the investigator.