Aberrant crypt foci (ACF) are putative precursor lesions of colon cancer, recently identified on the methylene blue-stained mucosal surface of human colon. No mutations in K-ras or p53 genes were found by non-radioactive single-strand conformation polymorphism analysis in 14 ACF collected from five patients. Using the more sensitive method of allele-specific polymerase chain reaction (PCR) for K-ras, 8 of 14 ACF were found to contain K-ras mutations, suggesting that mutated cells are present in minute clones in ACF. No dysplasia was observed in any of the ACF containing a mutated clone. The presence of K-ras mutations in ACF suggests that these lesions occur at a very early stage in human colorectal carcinogenesis.