To estimate diagnostic value of K-ras mutations during cancer risk group formation, they were studied in the samples of sporadic carcinomas (n = 33) and malignant (n = 13) polyps of large intestine obtained during surgery or polypectomy. Using PCR analysis, restriction analysis, SSCP analysis and automated sequencing, eight various point mutations were revealed. Six of them were located in codon 12 and two, in codon 13 of the K-ras gene. Mutation frequency in carcinomas, benign and malignant polyps was 43, 49, and 69%, respectively. In the healthy tissue of the large intestine, no changes in codons 12 and 13 in the K-ras gene were observed. Mutations in the groups of Russian patients examined partially overlapped. In patients with colorectal carcinoma the mutation frequency in the K-ras gene was not associated with disease onset age, location, and the extent of tumor differentiation while it was associated with the stage of tumor process. The maximum mutation frequency was revealed in polyps of patients over 70 years of age as well as in the adenomas of villous histology and large size ((1 cm). No correlation between the K-ras mutation frequency and the extent of polyp dysplasia was observed.