To understand the genesis of retinoblastoma from the perspective of tumor suppressor genes, especially the retinoblastoma (Rb) gene, 40 cases of retinoblastoma from 36 patients were investigated using polymerase chain reaction restriction fragment length polymorphism (PCR-RFLP) analysis of the p53, adenomatous polyposis coli(APC), and Rb antioncogenes. Loss of heterozygosity (LOH) affecting p53 was observed in 2 of 23 heterozygous cases (9%) and LOH of the Rb gene was observed in 17 of 20 heterozygous cases (85%). However LOH of the APC gene was not observed among 13 heterozygous cases. As expected, gene alterations of the Rb gene was most frequently observed and is regarded as a fundamental event in the development of retinoblastoma. Furthermore p53 and Rb gene alterations were observed simultaneously in two cases. Although it is not frequent, it is possible that p53 gene alterations in addition to Rb gene alterations may play a role in retinoblastoma development.