To evaluate the pattern of use of androgen deprivation therapy (ADT) with definitive radiotherapy (RT) in men with prostate cancer (PCa) in a population-based study in Australia.This is a prospective cohort of men with intermediate- and high-risk PCa, captured in the population-based Prostate Cancer Outcome Registry Victoria, who were treated with definitive prostate RT between January 2010 and December 2015. The primary outcome of interest was ADT utilization. Chi-squared test for trend was used to evaluate the temporal trend in the use of ADT over the study period. Multivariate logistic regressions were used to evaluate the effects of patient-, tumour- and treatment-related factors, and treatment institutions (public/ private and metropolitan/ regional) on the likelihood of ADT utilization.A total of 1806 men were included in the study, 199 of whom (11%) had favourable National Comprehensive Cancer Network (NCCN) intermediate-risk disease (i.e. only one intermediate-risk feature, primary Gleason grade 3, and <50% biopsy core involved), 687 (38%) had unfavourable NCCN intermediate-risk disease, and 920 (51%) had high-risk disease. Of the 1806 men, 1155 (64%) received ADT with RT. Men with NCCN high-risk PCa (84%) were more likely to have ADT than men with favourable NCCN intermediate-risk (32%) and unfavourable NCCN intermediate-risk (46%) PCa (P < 0.001). Men treated in public institutions (66%, vs 47% in private institutions; P < 0.001) and regional centres (78%, vs 59% in metropolitan institutions; P < 0.001) were more likely to receive ADT. There was a trend towards an increase in ADT utilization from 50% in 2010 to 64% in 2015 (P < 0.001). In multivariate analyses (adjusting for age, tumour-related factors, year of treatment and use of brachytherapy boost), treatment institution (public and regional) remained independently associated with increased likelihood of ADT utilization. Men with intermediate-risk PCa treated in regional and public institutions were 2.7 times (95% confidence interval [CI] 1.9-3.9; P < 0.001) and 2.8 times (95% CI 1.4-5.3; P = 0.002), more likely to receive ADT with RT, respectively, while men with high-risk PCa treated in regional and public institutions were 3.1 times (95% CI 1.7-5.7; P < 0.001) and 3.0 times (95% CI 1.7-5.4; P < 0.001), more likely to receive ADT with RT, respectively.This is the largest Australasian contemporary series reporting on the pattern of use of ADT with definitive prostate RT. While there was an increasing trend towards use of ADT over time, ADT still appeared to be underutilized in certain groups of patients who may benefit from ADT, with approximately one in five men with high-risk and one in two with unfavourable intermediate-risk PCa not receiving ADT with RT. There was notable variation in the use of ADT between public vs private and metropolitan vs regional institutions.