Background: While the addition of androgen deprivation therapy (ADT) to external beam radiotherapy is known to improve overall survival in Gleason 8-10 prostate cancer, it has been hypothesized that Gleason 9-10 disease, which is less differentiated than Gleason 8 disease, may be less sensitive to ADT. To investigate this idea, we examined the association between ADT and overall survival for Gleason 8 versus Gleason 9-10 prostate cancer. Methods: We identified 20,139 men in the National Cancer Database diagnosed with localized or locally advanced, Gleason 8-10 prostate cancer from 2004 through 2011 who received external beam radiotherapy. Patients with clinical evidence of nodal or metastatic disease were excluded. Cox proportional hazards regression was used to examine the association between ADT and overall survival. Results: Median follow-up was 4.0 years. 78.2% (9,509) of the 12,160 men with Gleason 8 disease and 86.6% (6,908) of the 7,979 men with Gleason 9-10 disease received ADT. On multivariable analysis, ADT was associated with a significant improvement in overall survival for Gleason 8 patients (adjusted hazard ratio 0.79, 95% confidence interval 0.71-0.88, P< 0.001) but not Gleason 9-10 patients (adjusted hazard ratio 0.96, 95% confidence interval 0.83-1.10, P= 0.532), with a significant interaction (P_interaction= 0.020). When considering Gleason 9-10 patients separately as Gleason 9 and Gleason 10, a higher Gleason score correlated with an increased adjusted hazard ratio for the association between ADT and overall survival (P_interaction= 0.012). Conclusions: In contrast to the significant survival advantage of ADT for Gleason 8 disease, our results strongly suggest that Gleason 9-10 disease may be less sensitive to ADT and that a higher Gleason score predicts lesser sensitivity. Consideration should be given to treatment intensification for Gleason 9-10 patients through enrollment in clinical trials or potentially adding novel antiandrogens or docetaxel, which have shown efficacy in both castration-resistant and castration-sensitive settings.