Based on the successful results achieved in survival and local control with radiotherapy of prostate cancer recent studies tried to establish some models to reliably predict late rectal toxicity. In fact, the rectum, due to its location, represents an organ at risk of acute and late toxicity with the onset of acute or chronic radiation proctitis. The concept of late consequential effect has gained ground. It implies that the late damage might be a direct consequence of the acute damage. Dose-escalation studies, conformal radiotherapy (3D-CRT) and intensity modulated radiotherapy (IMRT) led to the identification of parameters, based on dose-volume histograms (DVH), able to separate patients at low and high risk of toxicity. Precise predictive dosimetric factors play a major role in the definition of the onset of toxicity. The monitoring system of late toxicity used by the authors is presented.