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At present, pelvic phased array-coil MR is used as the validated imaging modality for measurement of the closest predicted radial mesorectal margin for rectal cancer. Endorectal ultrasound is also used to assess the clinical stage of the cancer that will determine the recommendation for neoadjuvant chemoradiation, but it has not been used to assess the closest predicted radial margin.We propose to assess endorectal ultrasound identification of mesorectal margins and the measurement of the closest predicted radial tumor-mesorectal margin.Patients included were those having MRI and endorectal ultrasound for evaluation of primary rectal cancer in 2010 at a tertiary cancer referral colorectal clinic. Clinical data, MRI, and endorectal ultrasound images were assessed. Two independent retrospective measurements of mesorectal dimensions were correlated to evaluate the reproducibility of identifying mesorectal margins. MRI and endorectal ultrasound images were compared for independent measurements of mesorectal dimensions and of the closest predicted radial mesorectal margin. MRI and endorectal ultrasound determination of margin involvement were assessed for agreement.Fifty-two patients were studied with an average rectal cancer distance to the anal verge of 6.8 cm. Interobserver correlation coefficients of endorectal ultrasound mesorectal dimensions ranged from 0.47 to 0.53 (p < 0.01). MR and endorectal ultrasound measurements of the closest predicted radial mesorectal margin were correlated r =0.56 (p < 0.0001). MR and endorectal ultrasound determination of margin involvement agreed in 81% of cases.Endorectal ultrasound has substantial agreement with MR to measure the closest predicted radial tumor-mesorectal margin. Correlations between observers and modalities for identification of mesorectal dimensions are modest. Further assessment is indicated to confirm endorectal ultrasound mesorectal measurements in a larger sample and to understand the advantages and disadvantages relative to MR.