Serrated adenoma of the colorectum was a newly proposed entity in 1990, characterized by epithelial neoplasia combining the architectural features of a hyperplastic (metaplastic) polyp with the cytological features of an adenoma. Its histogenesis and natural history still remain unclear. Forty-six serrated adenomas were obtained from 46 patients. The clinicopathological features were summarized. Paraffin-embedded blocks from 34 serrated adenomas were available for immunohistochemical studies using pS2, human gastric mucin, and p53 protein. Eighteen hyperplastic (metaplastic) polyps, 16 tubular adenomas, and 12 early-stage adenocarcinomas were randomly selected as control groups for immunohistochemical analysis. The patients' ages ranged from 32 to 86 (average 61โข4) years. Males were more frequently affected than females. Serrated adenomas were predominantly present in the left-side of the colon and in the rectum (72 per cent). Their sizes ranged from 3 to 26 mm (average 9โข2 mm). Six lesions (13 per cent) contained foci of high-grade dysplasia. These adenomas were significantly larger (12โข7 mm) than those containing no high-grade dysplasia (8โข6 mm). pS2 and human gastric mucin were expressed significantly more frequently in both hyperplastic (metaplastic) polyps and serrated adenomas than in tubular adenomas or adenocarcinomas. p53-positive cells were present in 18 of the 29 pure serrated adenomas (62 per cent) and in one of the five areas of low-grade dysplasia in serrated adenomas with high-grade dysplasia (20 per cent), most of which revealed a sporadic distribution. Only five of the 29 serated adenomas with no high-grade dysplasia (17 per cent) were regarded as demonstrating p53 overexpression. On the other hand, three of the five areas of high-grade dysplasia in serrated adenomas (60 per cent) revealed diffuse positivity (3+) for p53 protein. The serrated adenoma, which possibly shows gastric differentiation, is considered to be an independent histological entity among the various phenotypes of colorectal adenomas. Serrated adenoma would seem to be a precursor of carcinoma, its potential for malignant transformation being similar to that of the traditional tubular adenoma. It would also seem that p53 is involved in the serrated adenoma-carcinoma sequence.