The potential of flat-elevated colorectal adenomas to undergo rapid malignant transformation and progression to invasive carcinoma is still under discussion. Therefore, a total of 130 colorectal neoplastic lesions > or = 1 cm in diameter were examined after endoscopic or surgical resection. Lesions were macroscopically classified into three categories: (1) flat elevation (22 lesions), superficially elevated lesion with a smooth surface; (2) granular laterally spreading tumor (GLST; 26 lesions), laterally spreading aggregates of nodules forming a lesion with granular surface, and (3) polypoid (82 lesions), pedunculated, subpedunculated and sessile polyps. The adenomatous component showed a tubulovillous architecture in 9/26 (35%) of GLST and 18/82 (22%) of polypoid lesions, however none of the flat-elevated lesions had a villous component (p <0.01; p <0.05). Carcinoma was present in 17/22 (77%) flat elevations, 37/82 (45%) polypoid lesions and 11/26 (42%) GLST (p <0.05). None of the carcinomas arising in GLST and only 1/37 (3%) of those developing in polypoid lesions were invasive carcinomas, but 4/17 (24%) carcinomas arising in flat elevations showed submucosal invasion. Moreover, while all 5 noncancerous flat elevations showed severe atypia, 17/82 (21%) polypoid lesions and 5/26 (19%) GLST showed moderate atypia. In conclusion, flat-elevated colorectal neoplasms have a high malignant potential and the role of these lesions as precursors of colorectal carcinomas deserves greater emphasis.