Hereditary predisposition to retinoblastoma is caused by germline mutations in the RB1 gene. Most of these mutations occur de novo and differ from one patient to another. DNA samples from 10 families with a child presenting sporadic bilateral retinoblastoma have been analysed for the causative mutation. Using intragenic DNA polymorphisms we detected large deletions in two patients. Heteroduplex and DNA sequence analysis of PCR products from each exon and the promoter region showed small mutations in four patients: a C to T transition in exon 18; 1 bp and 2 bp deletion in exons 20 and 19 respectively; and a 4 bp insertion in exon 7. All these mutations are likely to result in premature termination of transcription. In one of these families, an unaffected carrier was detected. This emphasises the importance of detection of the causative mutation for predictive diagnosis in families with sporadic bilateral retinoblastoma.