The inheritance of a genetic susceptibility to the development of retinoblastoma generally follows an autosomal mode of inheritance with high penetrance. Rare families, however, show evidence of incomplete penetrance where individuals can transmit the mutant gene without being affected themselves. In these families formal proof of this dogma requires the identification of the predisposing mutation. In this study we have identified the mutations in cousins with bilateral (hereditary) disease. Using SSCP and DNA sequencing, different constitutional mutations were detected in the affected cousins in this pedigree. One cousin carries a C-->T mutation in exon 8 generating a stop codon directly which was also present in his affected mother whereas the other cousin carries an 8 base pair deletion in exon 20. Neither half of the family carried the same mutation as the other. The mother of the patient with the 8 bp deletion carried neither of the mutations. Thus, we have demonstrated that the retinoblastomas in this family have developed as a result of independent, sporadic genetic events which occurred coincidentally in the same extended family rather than being due to a common mutation which manifests as incompletely penetrant. These observations have important implications for genetic counselling in this type of family.