Objective：To study the diagnostic value of colonoscopy in screening of family members of a familial adenomatous polyposis （FAP） family. Methods： The colonoscopic findings of 38 sibs from 23 FAP families （1985-2002） were retrospectively studied. The diagnostic value of colonoscopy for screening FAP in FAP families was analyzed through determining the amount, morphology, and pathology of polyps. Results： Sixteen of the 38 sibs were found to have polypous growths, the positive screening rate being 42.1% ; of the 16 cases of polypous growths, one was found to be severe atypical proliferation. The left hemicolon and rectum had dense polyps. Conclusion： Colonoscopy is a safe and reliable screening examination in sibs of FAP families.