Adjuvant preoperative radiotherapy in patients with rectal cancer improves local control and possibly overall survival. However, an increased postoperative mortality rate after radiotherapy has been observed in some trials. This study was based on 1399 patients in two randomized trials of radiotherapy. It reviewed the causes of death after operation and attempted to identify risk factors for postoperative mortality in patients with rectal cancer treated with or without high-dose (5 x 5 Gy) preoperative radiotherapy. The majority of deaths were from cardiovascular disease or infection. The risk of postoperative mortality was significantly increased in patients irradiated with a two-portal technique to a relatively large volume compared with those not given radiotherapy, but not in those irradiated with a four-portal technique to a limited volume. Age, sex, tumour stage and coexistent cardiovascular disease were independent risk factors for postoperative mortality. The risk of postoperative death in patients with rectal cancer is related to the preoperative radiotherapy technique.