Objective: Several randomized controlled trials have documented significant overall survival benefit in high metastatic risk prostate cancer (PCa) patients treated with combination of androgen deprivation therapy (ADT) at radiotherapy (RT) relative to RT alone. Unfortunately, elderly patients are either not included or are underrepresented in these trials. In consequence, the survival benefit of combination of ADT at RT in the elderly warrants detailed reassessment, including its cost. Methods: Between 1991 and 2009 within the Surveillance Epidemiology and End Results (SEER)-Medicare-linked database, we identified 3,692 patients aged 80 years or more with clinical T1–T2 PCa and WHO histological grade 3, or clinical T3–T4 PCa and any histological grade, treated with or without combination of ADT at RT. Competing risks analyses focused on cancer-specific mortality (CSM) and other-cause mortality, after accounting for confounders. All analyses were repeated in patients with no comorbidity and in most contemporary patients, treated between 2001 and 2009. Finally, we assessed median annual cost according to use of combination of ADT at RT, after adjusting for patient and tumor characteristics. Results: In competing-risks multivariable analyses, no statistically significant difference was observed in CSM and other-cause mortality between patients treated with or without combination of ADT at RT. Same results were recorded in subgroup analyses of patients with no comorbidity and in most contemporary patients. The median annual costs of $36,140 and of $47,510 were recorded, respectively in patients treated without and with ADT at RT. Conclusion: Our findings failed to confirm that combination of ADT at RT reduces CSM rates in high metastatic risk PCa patients aged 80 years or more. Moreover, combination of ADT at RT resulted in a significant cost increase, relative to RT alone. © 2018 Elsevier Inc.