To investigate epithelial cell proliferation and oncoprotein expression of the serrated adenoma, a term that has been used synonymously with mixed hyperplastic and adenomatous polyp, immunohistochemical staining using polyclonal antibodies against Ki-67 and p53, and a Bcl-2 monoclonal antibody, was performed and the results compared with those in hyperplastic polyps and tubular adenomas. A total of 20 serrated adenomas all characterized by a serrated glandular pattern, contained immature goblet cells, upper crypt zone mitotic figures, and a few nucleoli within the epithelial cells. Twenty hyperplastic polyps and 20 tubular adenomas (all with low-grade dysplasia) were examined, and lesions that contained separate areas of hyperplastic and adenomatous glands were excluded. The Ki-67-positive rate in the middle zone of the crypts in serrated adenomas was significantly higher than in hyperplastic polyps but lower than in tubular adenomas; a similar tendency was also noted for the upper zone. Both serrated adenomas and hyperplastic polyps demonstrated Bcl-2-positive reactivity that was essentially limited to the lower crypt zone, while in contrast, involvement in tubular adenomas often extended to the middle zone. No p53 overexpression was found in any category. These results suggest that serrated adenomas may be committed to independent growth.