The gene for hereditary retinoblastoma (Rb), an autosomal dominant trait localized to the long arm of chromosome 13, is linked to the locus for the enzyme esterase D (EsD). We analyzed a three-generation family that demonstrates cosegregation of alleles at the EsD locus and the Rb locus. This kindred yields a logarithm of the odds ratio (LOD) score of 2.46 at a recombination fraction (0) of 0.0. When combined with five other recently reported families, the resulting maximum score was 11.08 at 0 = 0.0. This combined LOD score and the lack of demonstrable crossovers in more than 65 individuals indicate that predictions of the Rb gene carrier state based on EsD genotyping are at least 90% accurate.