Fifty patients were assessed for congenital hypertrophy of the retinal pigment epithelium (CHRPE) as a potential phenotypic marker for familial adenomatous polyposis (FAP), with and without other extracolonic manifestations (ECM). The ocular anomaly, which characteristically is multiple, benign, and congenital, was studied in three groups. Group 1 contained eight patients with nonpolyposis colon cancer as disease controls. All had negative eye findings. Group 2 included 40 patients with FAP, 35 (87.5 percent) of whom had retinal lesions. Twenty-two of 25 patients with FAP alone had retinal lesions while 13 of 15 patients with FAP and extracolonic manifestations were similarly affected. Group 3 included 11 offspring at risk for FAP. Eight (72.7 percent) offspring had retinal lesions. One of the eight subjects with the ocular trait was subsequently diagnosed with FAP. Two of the eight patients also had other ECM but have not been sigmoidoscoped for FAP. Seven of 11 offspring (mean age, 12.5 years) have had negative flexible sigmoidoscopy. Specificity of the retinal lesions in FAP cannot be ascertained until subsequent adenomas are identified on follow-up of the group at risk. The gene responsible for CHRPE appears to be transmitted from one generation to another, demonstrated by the high sensitivity of the retinal lesions in patients with FAP alone and with other ECM.