Radiotherapy has been used extensively as an adjuvant treatment with surgery for patients with rectal cancer. Present knowledge indicates that preoperative radiotherapy is more dose-efficient than postoperative radiotherapy in reducing local recurrence rate. Provided the dose is sufficiently high, the reduction exceeds 50% in all Dukes' stages and after both abdominal perineal excision and anterior resection. The effect on survival has not yet been proven but there are indications that survival may be slightly improved using preoperative radiotherapy, although the magnitude of this improvement is probably less than with postoperative chemotherapy. Of concern with all adjuvant treatments are the potential side-effects, and it appears that postoperative radiotherapy has more side-effects than preoperative radiotherapy, even if the reduction of local recurrences is less and proper radiation techniques are utilised.