Forty patients examined with the use of carbon dioxide insufflation during fiberoptic colonoscopy showed no significant residual gas on plain radiographs taken 30 minutes after the procedure. By contrast, 19 of 20 patients examined with air insufflation showed excessive distension of large and/or small intestine. The rapid absorption of carbon dioxide may be a safety factor in a few patients and adds to the comfort of the procedure in many others by virtually eliminating overdistension afterwards. It also makes it technically possible to perform a barium enema immediately after colonoscopy.