Although the retinoblastoma susceptibility locus (RB1) spans some 180 kb in the human and has been fully sequenced, few polymorphisms within the locus have been identified and none have been shown to vary in allelic frequency in different populations. We have identified a previously unknown polymorphism within intron 18 of the retinoblastoma susceptibility gene that is present in Asians but not in the other ethnic groups examined. This polymorphism eliminates a Tsp5091 restriction enzyme site, making it easily detectable for use in linkage analysis and genetic population studies.