The entire length of the colon, starting at the recto-sigmoid junction and ending at the cecum, can be visualized sonographically following retrograde water instillation into the colon. Using this method it is possible to evaluate in detail the lumen, the intestinal wall and the surrounding connective tissue. Severe active colonic Crohn's disease and ulcerative colitis can be detected by diagnostic sonography of the colon with a sensitivity of 91% and 89% respectively. Crohn's disease can be differentiated from ulcerative colitis in 86% of the cases by means of colonic sonography. Transabdominal sonography of the fluid-filled colon is a new diagnostic procedure especially in children with Crohn's disease and ulcerative colitis.