BACKGROUND A mutation in c-K-ras (KRAS2) has long been implicated as one of the important early events in the development of a large proportion of human colon cancers. Aberrant crypt foci, putative preneoplastic lesions identified microscopically in wholemounts of colons, have been shown to occur with high frequency in the colons of animals treated with colon carcinogens and in the grossly normal mucosas of patients with colon cancer. OBJECTIVE In this study, we asked whether the mutational activation of K-ras occurs in the aberrant crypt foci of human colon. METHODS Grossly normal colonic mucosas were obtained from seven patients during surgery and were provided to us by the Western Division of the Cooperative Human Tissue Network located at Case Western Reserve University. A total of 42 samples, consisting of aberrant crypt foci and similarly sized normal crypt areas, were microdissected from the grossly normal colonic mucosas. The DNA region containing codon 12 of K-ras was amplified by polymerase chain reaction and analyzed for mutations by dot-blot hybridization with specific oligonucleotide probes complementary to normal or mutant sequences. RESULTS Mutations in codon 12 of K-ras were found in 11 (73%) of 15 aberrant crypt foci but not in any of 27 morphologically normal crypt areas from the same patients. CONCLUSIONS The observed high frequency of K-ras mutations in these microscopically identifiable lesions makes mutation in K-ras the earliest identified gene-mutational event in human colon tumorigenesis, establishes that it often occurs prior to the development of polyps, and is consistent with the hypothesis that aberrant crypt foci are the earliest identified precursors of human colon cancer. CONCLUSIONS Further analysis of aberrant crypt foci may identify yet unknown early genetic events that precede human colon cancer.