Fifty-six patients have been examined in a prospective randomized study on the effects of air and carbon dioxide on post-procedural discomfort following colonoscopy. A significant reduction in post-procedural pain was observed at 6 hours (p = < 0.0005) and was still present the next day (p = 0.01). This was associated with a difference in the grading of flatus at 6 and 24 hours (p = < 0.0001 and < 0.05, respectively). An abdominal radiograph 1 hour after the procedure showed minimal gas in the CO2 patients, while the patients who had air showed distention of large and small bowel (p = < 0.0001 and < 0.01, respectively). Seventeen of 29 patients who had air suffered post-procedural pain, compared with 2 of 27 of the CO2 patients. Fifty-seven percent of the patients who were given air had colonic diameters over 6 cm on a 1-hour post-colonoscopy radiograph and 18% over 10-cm diameter. Provision by equipment manufacturers of simple and safe devices for routine delivery of CO2 for lower gastrointestinal endoscopy is long overdue.