Background: Many patients with advanced prostate cancer do not want to pursue continuous androgen deprivation therapy (cADT) because of concerns about side effects, and pursue iADT. However, even intermittent treatment with GNRH agonist therapy may not lead to rapid recovery of testosterone during the off-treatment period. In the pivotal HERO trial of relugolix (GNRH antagonist) the testosterone recovery time was in weeks [PMID 32469183]. The objective of the study was to measure the time to testosterone suppression to castrate level and time to recovery of serum testosterone to non-castrate level in real-world patients with advanced prostate cancer undergoing intermittent relugolix therapy. Methods: In this IRB-approved retrospective study, the eligibility was: patients with advanced prostate cancer undergoing iADT with relugolix. iADT with relugolix was defined as initiating relugolix at PSA ≥ 10 ng/ml and holding relugolix when PSA ≤ 4 ng/ml. The primary endpoints were: 1) time to serum castrate level of testosterone after relugolix initiation and 2) time to recovery to non-castrate level after discontinuation of relugolix. Castrate level of serum testosterone was defined as serum testosterone level ≤ 50 ng/dL. Results: Overall 25 consecutive patients with advanced prostate cancer treated with iADT with relugolix were eligible and included. The median age was 75 [range 68-88]. Patients with a Gleason score of ≥ 8 comprised 44% of the cohort. The median PSA at the start of therapy was 18.2 ng/ml [5.1 – 99.1 ng/ml]. Median time to testosterone ≤ 50 ng/dl after relugolix initiation was 1.13 months [0.67 – 2.5 months], and median time to testosterone recovery after relugolix discontinuation was 1.4 months [0.83 – 6.57 months]. The median PSA nadir achieved with relugolix was 2.02 ng/ml [0.1 – 4.8], and median time to PSA nadir was 1.35 months from initiation of therapy (0.67 – 4.8 months). Median time to PSA relapse ≥ 10 ng/dl after holding relugolix was 3.4 months (0.9 – 8.23 months). Conclusions: This is the first real-world study to show that iADT with relugolix is associated with a rapid time to testosterone suppression and recovery. These results may guide patients’ counseling, monitoring of serum testosterone and PSA levels in patients wishing to pursue iADT for advanced prostate cancer. These hypothesis-generating data need external validation.