Retinoblastoma is an intraocular malignancy of childhood, which is very radiosensitive. gamma-irradiation, however, induces side effects, and the precise mechanisms of tumor cell death after the treatment remain unknown. In this study, we demonstrated that gamma-irradiation induced apoptosis (programmed cell death) in human retinoblastoma cell lines Y79 and WERI-Rb-1. The expression levels of p53, tumor suppressor gene product, and its-associated protein, WAF1/CIP1, were both up-regulated, and function of p53 was remarkably activated after the treatment. Moreover, apoptosis was induced in the absence of gamma-irradiation by overexpression of the WAF1/CIP1 gene in both retinoblastoma cells. These results indicate that the transfer of the WAF1/CIP1 gene may have potential for the treatment of human retinoblastomas instead of gamma-irradiation.