BACKGROUND The magnifying colonoscope allows 100-fold magnified viewing of the colonic surface. METHODS We examined 2050 colorectal tumorous lesions by magnifying endoscopy, stereomicroscopy, and histopathology and classified these lesions according to pit pattern. Based on stereomicroscopy, lesions with a type 1 or 2 pit pattern were nontumors, whereas lesions with types 3s, 3L, 4, and/or 5 pit patterns were neoplastic tumors. RESULTS The pit patterns observed by magnifying endoscopy were fundamentally similar to those demonstrated in stereomicroscopic images. When the diagnosis by magnifying endoscopy was compared with the stereomicroscopic diagnosis, there was agreement in 1130 of 1387 lesions (81.5%). True neoplasms could be differentiated from non-neoplastic lesions. Of lesions with a type 5 pit pattern with a bounded surface, 11 of 22 (50%) were found to be invasive cancers with involvement of the submucosal layer. If this pit pattern is found to involve a relatively broad area of the mucosal surface, extensive malignant invasion (sm-massive) should be strongly suspected. CONCLUSIONS The magnifying colonoscope provides an accurate instantaneous assessment of the histology of colorectal tumorous lesions. This may help in decision making during colonoscopy.