Many patients of gastroenterology clinics complain symptoms requiring colonic investigations. Radiological examinations may fail to detect early inflammatory bowel diseases and small neoplasms or polyps. And therefore colonoscopies are performed in preference to barium enema. Recently several reports raised a question regarding whether biopsy is necessary in a macroscopically normal colon. To determine what proportion of apparently normal colons is histologically abnormal, to measure the thickness of subepithelial basement membrane(SEBM) in normal colons, and to see whether the thiekness varies according to the different areas of the large bowel we prospectively analyzed 100 consecutive subjects with normal laboratory findings and normal mucosa on colonoscopy. Significant histologic abnormalities were not detected in all 100 cases. Thickness of SEBM (mean+SD) was 1.6+0.4 um at hepatic flexure, 1.6+0.4um at splenic flexure and 1.7+0.4 pm at sigmoid colon. The SEBM was significantly thicker at the sigmoid colon than at the hepatic flexure. Range of thickness of normal SEBM was 0.8 to 2.5 pm(mean+2SD). Maximum thickness of SEBM was 3 pm. The result of this study suggests that doing colonoscopic biopsies in all normal colons do not seem to be essential in Koreans yet.