Colorectal carcinomas may be induced from adenomas, or they may occur de novo. To clarify the histogenesis of colorectal carcinomas, point mutations in codon 12 of the c-K-ras 2 gene in neoplasias of familial adenomatous polyposis patients were examined. Nineteen colorectal advanced carcinomas, 135 adenomatous polyps, 9 hyperplastic polyps, and 27 normal colonic mucosae were obtained from 48 patients. In 27 normal mucosae and 9 hyperplastic polyps, a mutation in the K-ras gene was not detected. Mutations were detected as follows: 0 of 24 in adenomas with mild atypia, 10 of 77 in adenomas with moderate atypia, and 24 of 34 in adenomas with severe atypia. The incidence of mutations in c-K-ras 2 codon 12 is correlated with the degree of atypia of adenomas. However, only 5 such mutations were detected in 19 advanced carcinomas, indicating that the mutation frequency in advanced carcinomas is much lower than that in adenomas with severe atypia. If a mutation of c-K-ras 2 gene is an important component in the formation of adenocarcinoma, these results did not confirm the successive development from adenomas with severe atypia to advanced carcinomas as the main route for colorectal carcinogenesis in familial adenomatous polyposis patients.