The ileocecal valve consists of an upper and lower segment formed by a duplication of the wall of the small and large bowels. The origin of tumor that occurs in the ileocecal valve is often difficult to prove. The tumor in the ileocecal valve was found with endoscopic examination in a 74-year-old man. The resected specimen of the tumor was genetically studied by using PCR-SSCP (polymerase chain reaction amplification, single strand conformational polymorphism) analysis to determine the origin and direction of the tumor. PCR-SSCP analysis of p53 showed a mutation of exon 8 only in the ileum side of the tumor. This suggests that the tumor has grown towards the ileum. The cancer in the ileocecal valve is derived from the colonic part of the valve which acquired the additional genetic change of p53 mutation.