Serrated adenoma has been recently proposed as a distinct histological lesion of the colorectum. This study examined p53 immunoreactivity, mutations of exons 5-8 of the p53 gene, codon 12 of the Ki-ras gene by PCR-SSCP analyses, and microsatellite instability in 19 serrated adenomas, ten adenocarcinomas in/with serrated adenomas, 23 hyperplastic nodules, four hyperplastic polyps and 29 tubular adenomas of the colorectum. Eleven of 11 (100 per cent) serrated adenomas had p53 immunoreactivity and all six (100 per cent) adenocacinomas in/with serrated adenomas exhibited moderate to severe p53 immunoreactivity. It was confirmed that 9 of 19 (47 per cent) serrated adenomas and 5 of 10 (50 per cent) adenocarcinomas in/with serrated adenomas harboured p53 gene mutations. On the other hand, no p53 gene mutation was detected in the other colorectal lesions. Meanwhile, 11 (58 per cent) serrated adenomas and six (60 per cent) adenocarcinomas in/with serrated adenomas had Ki-ras gene mutations, as also did 9 of 23 (39 per cent) hyperplastic nodules, 3 of 4 (75 per cent) hyperplastic polyps, and 12 of 29 (41 per cent) tubular adenomas. Microsatellite instability was detected in one (5 per cent) serrated adenoma and one (10 per cent) adenocarcinoma in a serrated adenoma. The other lesions did not show microsatellite instability. Serrated adenomas had significantly frequent p53 gene mutations compared with hyperplastic lesions or tubular adenomas (p < 0.005). On the other hand, they did not exhibit significant differences in mutations of the Ki-ras gene or in microsatellite instability. Genetic changes were then examined in small parts of serrated adenomas, such as the upper or lower parts of crypts, to determine the extent of gene mutations by using a microdissection technique. Exon 15 of the APC gene and the DCC gene, in addition to the p53 and Ki-ras genes and microsatellite instability, were analysed. Identical mutations of the p53 gene were found in both invasive adenocarcinomas and adjacent serrated adenomas by direct sequencing, suggesting single clonal origins for those lesions. Mutations of the APC gene and microsatellite instability were heterogeneous in some lesions. No loss of heterozygosity (LOH) of the DCC gene was found. These findings suggest that mutations of the p53 gene are the most characteristic genetic alterations in serrated adenomas, as a relatively early event in a multistep carcinogenic pathway of this type of colorectal lesion, that might be distinct from the ordinary adenoma-carcinoma sequence or from carcinogenesis via mutations of mismatch repair genes.