Objective. To investigate whether the use of carbon dioxide (CO2) compared with air insufflation during colonoscopy improves ultrasonography after the procedure. Material and methods. In a double-blind trial, 30 patients were randomized to insufflation with CO2 or air. Thirty minutes after colonoscopy abdominal ultrasound was performed. Immediately after ultrasonography, the ultrasound quality of the liver, gallbladder, biliary ducts, pancreas, spleen, kidneys, abdominal vessels, antrum, bowel, urinary bladder and prostate/uterus was evaluated for optimal ultrasound scanning quality, minor reduction of scanning conditions, major reduction of scanning conditions and unacceptable scanning conditions. Results. Ultrasound quality 30 min after colonoscopy was significantly better when using CO2 insufflation instead of air (p<0.003). Significant improvement in imaging quality was observed for the liver, portal vein, splenic vein, all three divisions of the pancreas, aorta, coeliac trunk, superior mesenteric artery, iliac vessels, left kidney and uterus. Conclusions.Ultrasound investigation can be done after a colonoscopy with CO2 insufflation, whereas it is not recommended after a colonoscopy with air insufflation. In selected cases, this approach may enable and improve post-colonoscopy ultrasound scanning.