A retrospective review of the familial adenomatous polyposis registry at the Cleveland Clinic Foundation revealed an incidence of spontaneous mutation in familial adenomatous polyposis (FAP) of 22 percent of family kindreds. These patients were reviewed retrospectively and compared with the total FAP population followed at The Cleveland Clinic Foundation with respect to the onset of disease, the incidence of carcinoma in the resected colon, and incidence of extracolonic manifestations. Review of the characteristics and presentations of these patients suggested that these individuals may harbor a more severe form of FAP. This may be due, in part, to the delay in diagnosis and, therefore, a higher rate of development of colorectal carcinoma and possibly duodenal adenomas. There is also a demonstrable higher rate of extracolonic manifestations of FAP present in this subset of patients. When selecting the initial type of prophylactic colonic resection the surgeon should bear in mind the increased incidence of extracolonic manifestations of the disease in this group of patients and their potential for complications.