A sensitive restriction fragment length polymorphism assay and DNA sequencing were used to detect c-Ki-ras mutations in 56 specimens of colonic epithelium from 18 patients with chronic ulcerative colitis. Mutations were not detected in biopsy specimens that were negative or indefinite for dysplasia. In 4 of 8 patients with high-grade dysplasia, a c-Ki-ras codon 12 or 13 mutation was detected. In three colectomy specimens, a wide area of dysplastic cells (greater than 10 cm2) contained a specific ras mutation. In two of these specimens, an invasive cancer contained a c-Ki-ras mutation identical to that found in adjacent dysplastic epithelium. These studies indicate that mutations of c-Ki-ras may be an excellent molecular genetic marker to map dysplastic fields and invasive cancer in ulcerative colitis.