We found, in an asymptomatic patient with familial occurrence of malignancy, that mutations in the oncogene Kras could be detected in stool 18 months before a premalignant polyp was detected and removed endoscopically. Colorectal cancers usually develop from benign adenomas in a lengthy period of 5-10 years. During this period, several major biochemical pathways are involved, each characterized by one or several genetic alterations. Our patient did not present any signs or symptoms of colorectal disease during his two visits to the endoscopist. This case report shows that the use of genetic markers in stool testing has the potential to detect colon cancer in its very early stages when treatment is simple and often successful.