Serrated adenomas are the precursors of at least 5.8% of colorectal cancers; otherwise little is known of their clinical significance in comparison with conventional adenomas and hyperplastic polyps. We compared the risk of metachronous lesions in colorectal serrated adenomas, conventional adenomas, and hyperplastic polyps. A consecutive series of patients with colorectal polyps first diagnosed from January 1978 to December 1982 and follow-up specimens to the end of 2000 was reviewed, and 239 polyps fulfilling the selection criteria were chosen as index polyps. The type of polyp seen in follow-up correlated significantly with the type of the initial lesion. Serrated adenomas were estimated to grow faster than conventional adenomas, but the incidence of colorectal cancer did not differ significantly between serrated (2/38 [5%]) and conventional adenomas (2.2%). The results indicate that serrated adenomas are lesions with a significant risk of metachronous serrated adenomas and the development of cancer. We emphasize the need for the proper recognition and management of serrated adenomas.