Gastrointestinal symptoms after pelvic radiotherapy, which affect quality of life, are substantially more common than generally recognised and are frequently poorly managed. These symptoms develop because radiation can induce change in one or more specific physiological functions in widely separated parts of the gastrointestinal tract that lie in the path of the radiotherapy beam. Radiation-induced changes are not confined by normal anatomical boundaries. Furthermore, pre-existing subclinical disease might be destabilised because of minor gastrointestinal changes induced by radiotherapy. New diseases might manifest after radiotherapy and be confused with symptoms induced by radiotherapy. Different functional deficits might cause the same symptoms. Many patients have more than one cause for their symptoms, which sometimes need very different treatments. Simple diagnostic tests that are used in other contexts, if applied appropriately to patients with new gastrointestinal symptoms after radiotherapy, can identify the underlying causes of new-onset symptoms. Starting treatment without knowing the cause of the symptom is commonly ineffective because prediction of the combination of treatments needed is difficult. Evidence suggests that many patients have unusual but highly treatable malfunctions of gastrointestinal physiology, which if correctly diagnosed may enable a patient with difficult symptoms to be helped.