Background: To properly balance the benefit (reduction of local recurrence) of short-term pre-operative radiotherapy for resectable rectal cancer against its harm (complications), a consensus concerning the severity of complications is required. The aim of this study was to reach consensus regarding major and minor complications after short-term radiotherapy followed by total mesorectal excision in the treatment of rectal carcinoma, using the Delphi technique. Methods: A Delphi round was performed in cooperation with 21 colo-rectal surgeons from the Netherlands, United Kingdom and Sweden. The key-question was: 'Which of the predefined complications, caused or substantially aggravated by radiotherapy, are so important (major) that they might lead to the decision to abandon short-term pre-operative radiotherapy (5 × 5Gy) when treating patients with resectable rectal cancer (T1-3N0-2M0)?' Results: After three rounds, consensus was reached for 37 (68%) of 54 complications of which 13 were considered major and 24 considered minor. The following complications were considered to be major: mortality, anastomotic leakage managed by relaparotomy, anastomotic leakage resulting in persisting fistula, postoperative haemorrhage managed by relaparotomy, intra-abdominal abscess without healing tendency, sepsis, pulmonary embolism, myocardial infarction, compartment syndrome of the lower legs, long-term incontinence for solid stool, long-term problems with voiding, pelvic fracture with persisting pain, and neuropathy with persisting pain (legs). Three of 17 complications without consensus showed a tendency to be considered as major: perineal wound dehiscence managed by surgical treatment, small bowel obstruction leading to relaparotomy and long-term incontinence for liquid stool. Conclusion: The 13 major and three 'accepted as major' complications can be used to properly balance the benefit and harm of short-term pre-operative radiotherapy in resectable rectal cancer. This may eventually lead to improved treatment strategies for these patients.