Genetic counselling in retinoblastoma: importance of ocular fundus examination of first degree relatives and linkage analysis. We report an unusual family pedigree segregating the retinoblastoma predisposition gene. Expression of the phenotype in different individuals in this family ranges from asymptomatic gene carriers, regressed tumours, through unifocal to bilateral multifocal lesions. Because of the unusual pattern of inheritance in this family, initial genetic counselling at a local hospital did not take into account the possibility of incomplete penetrance of the gene, and complete ophthalmological examination of unaffected family members was not undertaken. We have used DNA probes from within the retinoblastoma predisposition gene for unequivocal identification of gene carriers. The subsequent demonstration of regressed tumours in founder members of the family confirmed the diagnosis of a dominantly inherited disease. The circumstances of the management of this family emphasises the need for specialist ophthalmic examination of first degree relatives and detailed genetic analysis of all such families with DNA probes. 