The radiosensitizing effect of platinum compounds has been demonstrated in a number of tumors. In prostate cancer, clinical and preclinical data concerning an eventual efficacy of the concept of radiosensitization are lacking. In the present study cisplatin and carboplatin have been used as a model to explore radiosensitization in in vitro prostate cancer cell lines. Human (DU-145) and rat (R3327-MATLyLu) prostate tumor cells were irradiated with doses ranging from 0 to 8 Gy in the presence of various concentrations of either cisplatin or carboplatin. For the evaluation of the combined effect of the two treatment modalities, a simple model is presented. Supra-additive treatment effects of combinations of platinum drugs with radiotherapy, both at clinically achievable doses, were shown on the basis of surviving fractions of tumor cells and proved to be significant. These data strongly suggest that radiotherapy may be effectively combined with radiosensitizers such as platinum drugs in prostate cancer therapy, to yield synergism in treatment efficacy.