To study the significance of detection of K ras mutation in stool samples in the diagnosis of large intestinal cancer, stool and local tissue samples were collected from consecutively diagnosed patients, among them 23 cases suffered from large intestinal cancer, 20 cases from colorectal adenomatoid polyps and 20 healthy subjects. DNA was extracted from both the stool and the tissue, K ras gene mutations at the first and second base of codon 12 were studied by allele specific mismatch method in both of them. The results showed that K ras mutation rate in large intestinal cancer was 56 52%(13/23), which was much higher than 5%(1/20)in healthy subjects( P 0 05). There was a 92 31%(12/13)incidence of K ras mutation at the second base of codon 12 in large intestinal cancer. The results of detecting the K ras mutation were highly identical in both the stool and the tissue ( P