BACKGROUND AND PURPOSE: Late rectal complications are assessed according to different scoring systems. Endoscopy can provide a more sensitive estimation of early radiation damage. The aim of this paper is to investigate the correlation between dosimetric parameters and rectal mucosal changes after radiotherapy (RT). MATERIALS AND METHODS: Patients with prostate adenocarcinoma treated with curative or adjuvant RT underwent endoscopy 1year after RT. Receiver operating characteristics (ROC) analysis was performed to analyze the predictive capability of the dosimetric variables in determining mucosal changes classified by Vienna Rectoscopy Score (VRS). RESULTS: The best dosimetric predictors of grade ⩾2 telangiectasia were rectal (r) V(60 Gy) (p=0.014), rV(70 Gy) (p=0.017) and rD(mean) (p=0.018). Similar results were obtained for grade ⩾2 VRS. The set of rV(60 Gy)<34.4%, rV(70 Gy)<16.7% and rD(mean)<57.5Gy was associated with a decreased risk of grade ⩾2 telangiectasia and VRS. CONCLUSIONS: rV(60 Gy), rV(70 Gy) and rD(mean) were the strongest predictors of rectal mucosal alterations. In-depth analysis is required to correlate each mucosal alteration with late rectal toxicity in order to suggest early proctoscopy as surrogate end-point for rectal late toxicity in studies aimed at reducing this important complication.