Retinoblastoma, a child tumor of the eye, is caused by two mutational events at the retinoblastoma gene (RB1). Retinoblastoma occurs in both hereditary and nonhereditary forms, and this distinction has important implications for patients and their families. In most patients with isolated unilateral retinoblastoma, tumor development is initiated by somatic inactivation of both alleles of the RB1 gene. Some of patients with hereditary retinoblastoma initially present with unilateral disease, and up to 10% to 12% of these patients only express unilateral disease. Germline mutation in RB1 gene confer hereditary predisposition to retinoblastoma. This study was designed to identify germline mutations in RB1 gene in Korean retinoblastoma patients. Samples of peripheral blood were obtained from 5 patients with isolated unilateral tumors. To detect genetic alteration in RB1 gene, exon 8, 10, 11, 14~20, 22 and 23 were investigated by PCR -SSCP. Bandshifts on SSCP were found in three out of 5 patients at exon 8. There were same point mutations from CGA (arginine) to TGA (stop codon) at codon 251 in exon 8 of RB1 gene. This point mutation has not been found in Korean patient with retinoblastma. But it is common mutation on the Western reports and Korea 's annual incidence of this tumor is similar in proportion to that of the West. Therefore, if a lot of patients are investigated to elucidate RB1 mutation this point mutation will be found. Identification of the germline mutation in RB1 gene would help to improve the presymptomatic diagnosis and clinical management to retinoblastoma patients.