We studied prospectively the utility of congenital hypertrophy of the retinal pigment epithelium as a predictor of colonic polyposis in offspring of patients with familial adenomatous polyposis with extracolonic manifestations (Gardner's syndrome). After they underwent initial indirect ophthalmoscopy, we followed up 34 individuals at 50% genetic risk for familial adenomatous polyposis with extracolonic manifestations due to an affected parent. All 34 obtained their first colorectal endoscopic examination during a follow-up period of up to 4 years. The 16 individuals who did not have congenital hypertrophy of the retinal pigment epithelium (aged 13 to 40 years; mean, 25 years) remained polyp free, while 14 of 18 individuals with congenital hypertrophy of the retinal pigment epithelium (aged 9 to 30 years; mean, 18 years) were found to have colorectal adenomatous polyposis. Our findings indicate that the presence of multiple patches of congenital hypertrophy of the retinal pigment epithelium in children and young adults at risk for familial adenomatous polyposis with extracolonic manifestations is a clinically useful predictor of colorectal polyposis.