Preoperative radiotherapy substantially lowers local failure rates after rectal cancer surgery, an effect that is seen whether surgery is optimized, total mesorectal excision (TME), or not. Preoperative radiotherapy also slightly improves survival. Postoperative radiotherapy also decreases local recurrence rates, but the relative reduction is less pronounced even when higher doses are used. Preoperatively, a high-dose, short-term schedule, 5 x 5 Gy in one week, has been used in several randomised trials. This is a convenient schedule and has low toxicity if properly conducted. In trials where the radiation technique has been simplified, resulting in large irradiated volumes, unacceptable acute and late toxicities have been seen. It has not yet been possible to detect any late toxicity in trials where the treatment was given using techniques avoiding the irradiation of unnecessarily large tissue volumes outside the target volume. The relative advantages of using this short-term schedule relative to a conventional irradiation for about 5 weeks are discussed.