Risk estimation for siblings or offspring is important in genetic counseling of patients with hereditary retinoblastoma. The RB1 gene spans approximately 200 kb in length, containing 27 exons. The use of polymorphic markers within the RB1 gene will eliminate the need of laborious specification of a mutation. The present study determined types and frequencies of VNTR polymorphisms of the 17th and 20th introns of the RB1 gene in 50 unrelated Japanese, using PCR amplification. In the 17th intron VNTR, there were 4 alleles, which ranged from 1400 by to 1550 bp. The most common allele was 1400 bp with a frequency of 73%, and the hetetozygosity rate was 46%. In the 20th intron VNTR, there were at least 9 alleles, wihch ranged from 192 bp to 240 bp. The alleles were more evenly distributed than those of the 17th intron VNTR, and the heterozygosity rate was 64%. These VNTR polymorphisms were successfully applied to the prediction of retinoblastoma and to the determination of parental origin of a chromosome deletion in 3 families with hereditary retinoblastoma. Analysis of VNTR polymorphisms within the RB1 gene proves to be practical and efficient for risk estimation in hereditary retinoblastoma.