Colorectal cancer is one of the most common malignancies in Norway. Many cases of the disease are detected at a stage where surgery is unlikely to result in cure. Currently used screening tests based on faecal occult blood and other tumour markers are poor indicators of colorectal neoplasia. Multiple gene alterations are associated with colorectal carcinogenesis. Mutations in the Ki-ras oncogene occur in 50% of colorectal carcinomas and also in 50% of the precursor lesions, the adenomas. These mutations have proved to be detectable in the faeces of patients with colorectal tumours, but the techniques used so far have been impractical for screening purposes. We have developed a rapid and simple laboratory technique, based on the polymerase chain reaction, for detecting mutated Ki-ras in the faeces. In eight of 12 patients with mutated Ki-ras in adenomas or carcinomas we found the corresponding mutations in stool samples. Our results represent a significant progression towards a simple and effective DNA-based screening strategy for early detection of curable colorectal cancer.