The irritable bowel syndrome (IBS) is a common disorder estimated to affect up to 20% of adult Caucasians, with only a small percentage requiring investigation. This prospective study was performed to substantiate the impression that patients with IBS experience more pain during endoscopic examination of the colon than do patients with other conditions. Patients with IBS were observed to experience significantly more pain during colonoscopy than did patients without IBS (median observed pain scores 46 and 9, respectively), p < 0.001. The intensity of the pain perceived during examination was significantly higher for patients with IBS than for those without IBS (median 52.5 and 23.5, respectively), p < 0.001. Within the groups, there was no significant difference between flexible sigmoidoscopy and colonoscopy in observed or perceived pain. 64% of the patients with IBS said that the pain experienced at colonoscopy was identical to their presenting pain. This study supports the hypothesis of a lower colonic pain threshold with colonic hyperalgesia in patients with IBS. We have found that hypersensitivity to the endoscopic examination of the colon is a useful clinical adjunct in the diagnosis of the IBS in those selected to undergo colonoscopy.