UNLABELLED Endorectal ultrasound is a well-established method of preoperative staging of rectal neoplastic lesions. PURPOSE This study was undertaken to evaluate whether tumor site (in terms of height) and position (with respect to the rectal circumference) have an influence on the reliability of endoluminal ultrasound staging. METHODS From January 1991 to May 1996, 154 consecutive patients with a total of 162 rectal tumors were examined preoperatively using endorectal ultrasound. Apart from staging all tumors using the uT/uN classification, tumor level and tumor position were recorded prospectively. Neoplasms were subdivided into low rectal (0-6 cm from the anal verge), mid rectal (7-12 cm), and higher lesions (> 12 cm). Furthermore, the lumen was divided into an anterior, left lateral, posterior, and right lateral position, and all tumors, apart from circular lesions (n = 9), were subclassified accordingly. RESULTS Overall, we found 40 (25 percent) adenomas, 15 (9 percent) T1, 29 (18 percent) T2, 67 (41 percent) T3, and 11 (7 percent) T4 lesions. Overall accuracy was 78 percent. Staging accuracy for low rectal tumors (n = 41) was 68 percent, whereas 76 and 88 percent of mid (n = 96) and high (n = 25) neoplasms were staged correctly, respectively. The difference was not statistically significant. With regard to position, 47 tumors were situated anteriorly (77 percent accuracy), 42 in the left lateral position (69 percent accuracy), 33 posteriorly (73 percent accuracy), and 31 in the right lateral position (81 percent accuracy). Differences did not reach statistical significance. CONCLUSION Endorectal ultrasound is currently the best method for preoperative assessment of the depth of infiltration of rectal tumors. However, rectal anatomy seems to affect staging accuracy in the lower rectum because the structure of the ampulla recti renders endosonographic examination more difficult. In addition, endosonographic layers are less well defined at this level. Both factors contribute to a lower reliability and predictive value of endorectal ultrasound staging in the lower rectum, although statistical significance was not reached in this study. On the other hand, tumor position with respect to rectal circumference does not influence the predictive value of endorectal ultrasound.