We have carried out presymptomatic prediction of mutant gene carrier status in ten individuals with a family history of retinoblastoma. In all cases standard linkage studies were employed using intragenic DNA probes which recognise restriction fragment length polymorphisms. In four cases foetal DNA samples were obtained by chorionic villus sampling, the remaining six were derived from either cord blood samples or venipuncture of neonates. We demonstrated that the mutant gene was inherited by only one of these patients who has subsequently developed bilateral tumours. Six of the other cases have now reached the age beyond which it might have been expected that tumours would develop and are all disease free. It must be concluded that repeated ophthalmological examination of these and future patients shown not to have inherited the mutant gene, is unnecessary.