Retrograde instillation of water into the colon makes it possible to visualize the colon sonographically from the rectosigmoid transition to the cecum in 97% of the patients examined. The sonographic views obtained using this technique, which is called hydrocolonic sonography, show the echo-free intestinal lumen, the five individual layers of the colon wall and the connective tissue surrounding the colon. Colonic polyps and carcinomas appear sonographically as echogenic structures projecting from the intestinal wall into the lumen. Colonic polyps larger than 7 mm diameter can be identified in 91% of cases, and the sensitivity of the technique in the detection of carcinomas of the colon is 97%. In addition, detailed evaluation of the structure of the bowel wall permits more precise staging of tumors of the colon. Aside from such localized changes, hydrocolonic sonography can also demonstrate typical changes in the wall structure in chronic inflammatory large-bowel diseases. In acute Crohn's disease the normal stratified appearance of the colonic wall is no longer in evidence and the wall appears visible thickened. In contrast, in patients with acute ulcerative colitis the normal sonographic stratified appearance of the colonic wall is maintained. Hydrocolonic sonography enables a detailed sonographic examination of the colonic lumen and the colon wall, thus providing additional information and allowing more precise diagnosis of many diseases of the colon.