In retinoblastoma, genetic alteration of N-myc amplification different from the alteration of the RB1 gene on chromosome 13q14 has been described. This study is to determine the frequency of N-myc amplification by fluorescence in situ hybridization method in retinoblastoma. This study was prospectively derived from 26 patients who were diagnosed as having unilateral retinoblastoma (highly progressive large retinoblastoma, group 5 in Reese-Ellsworth classification) and underwent enucleation. We performed locus-specific fluorescence in situ hybridization probes for N-myc gene. Our results demonstrated that in only one of 26 patients was N-myc amplification found in retinoblastoma tissue. N-myc amplification has been regarded as one characteristic of retinoblastoma cell line and an adverse prognostic factor. However, our study indicates that N-myc amplification is not frequently found in retinoblastoma.