Penetrance and segregation rates of mutant Rb-1 alleles were assessed in all 51 members of eight kindreds with hereditary retinoblastoma by concomitant ophthalmologic examination and determination of seven intragenic restriction fragment length polymorphisms (RFLPs). Penetrance was in the range reported in the literature except for one family in which it was only 42.8%. However, the odds of transmitting a mutant Rb-1 allele from one generation to the next were 25:9 in this population, much above the Mendelian 1:1 ratio (P less than 0.025). This preferential transmission was discovered through the use of molecular information. Further analysis revealed that this distortion was due to preferential inheritance among children of male carriers (18:4, P less than 0.005). No difference from a 1:1 segregation ratio could be detected among the children of female carriers (7:5). These findings were consistent with a review of relevant data in the literature.