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Purpose: To identify dosimetric parameters derived from anorectal, rectal, and anal wall dose distributions that correlate with different late gastrointestinal (GI) complications after three-dimensional conformal radiotherapy for prostate cancer. Methods and Materials: In this analysis, 641 patients from a randomized trial (68 Gy vs. 78 Gy) were included. Toxicity was scored with adapted Radiation Therapy Oncology Group/European Organization for the Research and Treatment of Cancer (RTOG/EORTC) criteria and five specific complications. The variables derived from dose–volume histogram of anorectal, rectal, and anal wall were as follows: % receiving ≥5–70 Gy (V5–V70), maximum dose (Dmax), and mean dose (Dmean). The anus was defined as the most caudal 3 cm of the anorectum. Statistics were done with multivariate Cox regression models. Median follow-up was 44 months. Results: Anal dosimetric variables were associated with RTOG/EORTC Grade ≥2 (V5–V40, Dmean) and incontinence (V5–V70, Dmean). Bleeding correlated most strongly with anorectal V55–V65, and stool frequency with anorectal V40 and Dmean. Use of steroids was weakly related to anal variables. No volume effect was seen for RTOG/EORTC Grade ≥3 and pain/cramps/tenesmus. Conclusion: Different volume effects were found for various late GI complications. Therefore, to evaluate the risk of late GI toxicity, not only intermediate and high doses to the anorectal wall volume should be taken into account, but also the dose to the anal wall. [Copyright &y& Elsevier]