BACKGROUND To investigate very early lesion of colorectal cancer, K-ras activation and nuclear p53 accumulation were studied in aberrant crypt focus (ACF). METHODS ACF were microscopically identified in grossly normal mucosa of patients with colorectal cancer who underwent surgery. Each ACF was microdissected from the surgical specimen and divided into two pieces, one for histologic and immunohistochemical examinations and the other for K-ras activation. K-ras mutations in codons 12 and 13 were sequenced after being screened by polymerase chain reaction amplification followed by restriction fragment length polymorphism analysis. Intranuclear accumulation of p53 protein was immunostained with the avidin-biotin complex method. RESULTS ACF was predominantly distributed in the sigmoid colon and rectum, and its incidence was increased with age. Unexpectedly, ACF was very rare in colons of three patients with hereditary nonpolyposis colorectal carcinoma. K-ras mutations were detected in 58% (33 of 57) of ACF cases and in 44% (11 of 25) of adenocarcinoma cases. Although GTT mutation in codon 12 was predominantly observed in adenocarcinoma (10 of 11), GAT mutation (12 of 33) was as frequent as GTT mutation (11 of 33) in ACF together with mutation at codon 13 (7 of 33). No accumulation of p53 protein was found in any ACF. CONCLUSIONS ACF were not diagnosed as neoplasms histologically, but they were considered to be neoplastic lesions, and K-ras activation is one of the key events to form ACF. The G-T substitution in K-ras codon 12 may undergo malignant growth easily compared with G-A substitution in colorectal carcinogenesis.