Abstract Purpose Several reports have shown that cells with p53 mutations display increased resistance to ionizing radiation, a treatment often used clinically for localized prostate carcinoma. Materials and Methods Totals of 18 post-irradiated locally recurrent prostatic carcinoma specimens and 25 (no radiation) stage D1 node-positive (TxN+MO) primary prostatic carcinoma specimens were tested for p53 immunoreactivity by immunohistochemistry. Of the 18 post-radiation locally recurrent prostatic carcinomas 10 were further analyzed by single strand conformational polymorphism to assess the validity of using this immunohistochemistry approach in irradiated tissue for detecting p53 alterations. Specimens showing p53 alterations by single strand conformational polymorphism were subjected to nucleotide sequence analysis or tested for loss of heterozygosity at a locus within the p53 gene. Results Of the 25 stage TxN+MO prostatic carcinomas without radiation 5 (20 percent) were immunoreactive (consistent with the reported incidence of positive immunoreactivity in clinical/surgical stage TxN+MO primary prostatic carcinomas). In contrast, 13 of 18 post-radiation locally recurrent prostatic carcinoma specimens (72 percent) were immunoreactive. Multivariate logistic regression analysis showed no dependence of p53 immunoreactivity to grade, stage or androgen status in the post-radiation locally recurrent prostatic carcinoma group, while 8 of 10 hormone naive prostatic carcinoma specimens (80 percent) were immunoreactive. The temporal relationship between p53 alterations and radiotherapy was assessed. Pre-irradiation prostatic carcinomas available from 5 patients with immunoreactive post-radiation locally recurrent disease were analyzed and all were immunoreactive. Conclusions p53 Alteration in localized prostatic carcinoma is uncommon. Our study confirms others in that even aggressive locally advanced nonirradiated primaries (stage TxN+MO) contain only a 20 percent incidence of p53 alterations. However, our study demonstrates that p53 alterations are found in the preponderant majority of post-radiation locally recurrent prostatic carcinoma specimens. Limited evaluation of pretreatment prostatic carcinoma biopsies uniformly documented the presence of p53 alterations before ionizing radiation, thereby demonstrating that p53 alteration was already present and was not radiation-induced or only correlated with late stage disease. This finding suggests a potential for p53 immunoreactivity to be used as a pretreatment marker that might predict local treatment failure with ionizing radiation. Large scale prospective trials would appear warranted to evaluate conclusively the potential prognostic applicability of p53 pre-screening before enrollment in definitive radiotherapy.