OBJECTIVE To clarify whether mucosal crypt patterns observed with magnifying colonoscopy are feasible to distinguish non-neoplastic polyps from neoplastic polyps. METHODS From June 1999 through March 2000, 180 consecutive patients with 210 lesions diagnosed with a magnifying colonoscope (CF-200Z, Olympus Optical Co., Ltd., Tokyo, Japan) were enrolled. Magnification and chromoendoscopy with 0.2% indigo-carmine dye was applied to each lesion for mucosal crypt observation. Lesions showing types I and II crypt patterns were considered non-neoplastic and examined histologically by biopsy,whereas lesions showing types III to V crypt patterns were removed endoscopically or surgically. The correlation of endoscopic diagnosis and histologic diagnosis was then investigated. RESULTS At endoscopy, 24 lesions showed a type I or II pit pattern, and 186 lesions showed type III to V pit patterns. With histologic examination, 26 lesions were diagnosed as non-neoplastic polyps, and 184 lesions were diagnosed as neoplastic polyps. The overall diagnostic accuracy was 99.1% (208/210). The sensitivity and specificity were 92.3% (24/26) and 99.8% (184/186), respectively. CONCLUSIONS Magnifying colonoscopy could be used as a non-biopsy technique for differentiating neoplastic and non-neoplastic polyps.