In order to lower unacceptably high local failure rates after surgery reported as curative for rectal cancer, perioperative radiotherapy has been extensively investigated. The collected information from a number of controlled trials indicates that the proportion of local recurrences is reduced to less than half when radiotherapy at moderately high doses is given preoperatively. This reduction in local failure rates is not seen after postoperative radiotherapy, even if higher doses have been used. Possibly, there is also a slight positive influence on survival from preoperative radiotherapy. Improved survival has been seen also in trials using postoperative radiotherapy, but then only when combined with chemotherapy. With proper dose planning, sufficiently high doses can be given preoperatively with little if any increase in postoperative mortality. Thus, although published knowledge is still rather limited, a properly planned preoperative radiotherapy seems to inflict small bowel and other complication rates, that are less than when radiotherapy of similar efficacy against the tumour is given postoperatively.