The interfamilial diversity in penetrance and expressivity of hereditary retinoblastoma was investigated in 29 families. By using a simple parameter for estimating the severity of the disease (diseased-eye-ratio), we were able to identify four families with a discrete low-penetrance phenotype. The underlying genetic defect was identified in three families. One family has a 3-bp deletion in exon 16 that results in the deletion of Asn480. In two further unrelated families, the identical missense mutation at codon 661 in exon 20 (CGG to TGG, Arg to Trp) was identified. These mutations are distinct from the majority of retinoblastoma gene alterations, as they do not result in the disruption of the gene product. We propose that reduced penetrance of retinoblastoma is the result of a residual function of these alleles in retinoblastoma precursor cells.