A study was carried out to evaluate congenital hypertrophy of the retinal pigment epithelium (CHRPE) as a disease marker in a defined population with familial adenomatous polyposis (FAP). Indirect ophthalmoscopy was performed on 75 individuals from 25 known families with FAP, of whom 32 were known to be affected and 43 were at a 50 per cent prior risk of developing the disease. A further ten individuals from five families with hereditary non-polyposis colorectal cancer (HNPCC) were also tested. CHRPE was seen in 28 of the 32 affected individuals, 27 of whom met the criteria for a positive examination. Three individuals at risk of FAP also had positive examinations. Five individuals from the families with HNPCC also had CHRPE, although none met the criteria for a positive examination. Of four types of CHRPE analysed, one (small pigmented dots) was found to be more frequent in older family members (P = 0.012), suggesting that this type of lesion may proliferate with age. Compliance with ophthalmic screening was 97 per cent in families with FAP. Using a combined set of diagnostic criteria, CHRPE identified affected individuals with a specificity of at least 94 per cent and a sensitivity of 84 per cent. Results argue for a combined screening programme for FAP of DNA analysis, indirect ophthalmoscopy and bowel examination.