Background: Leuprolide is a frequently used luteinizing hormone-releasing hormone (LHRH) agonist for the delivery of androgen deprivation therapy (ADT) for prostate cancer (PCa). Evidence suggests achieving and sustaining testosterone (T) levels at <20ng/dL with ADT is desirable and correlates with improved disease-specific survival in patients with advanced PCa. However, T levels may rise above castration level (50ng/dL) between injections, especially if a subsequent dose is delayed. Increase in prostate-specific antigen (PSA) levels on ADT may represent true disease progression to castration-resistant PCa, or may be a result of late ADT dosing and inadequate T suppression. This study evaluated the timeliness of leuprolide dosing, subsequent rate of T breakthroughs above 20ng/dL and frequency of T/PSA tests prior to dosing in PCa patients. Methods: A retrospective review of electronic medical records and associated claims data (1/1/07-6/30/16) of leuprolide injections (n=78,464) evaluated the frequency of late dosing (defined as occurring after day 32, 97, 128, 194 for 1-, 3-, 4-, 6-month formulations, respectively), T tests >20ng/dL and frequency of T/PSA tests prior to dosing. Results: Of all leuprolide injections, 26.8% of subsequent injections were late: 14.4% were ≤1 week, 3.2% were between 1-2 weeks and 9.1% were >2 weeks late. 42% of T values exceeded 20ng/dL for late injections; while only 21% exceeded this level for early/on-time injections. 83% of leuprolide injections had a PSA value drawn prior to dosing; only 14% had a similarly timed T assessment. Conclusions: Overall, greater than a quarter of injections were late. When leuprolide dosing was late, the proportion of T tests with T >20ng/dL was higher compared to when the dosing was early/on-time. Late injections were correlated with ineffective T suppression (above 20ng/dL) over 40% of the time. For all injections, T levels were not monitored as frequently as PSA levels. Considering the clinical benefits of maintaining effective T suppression throughout the course of ADT, clinicians should ensure treatments are delivered within approved dosing instructions, T levels are routinely monitored and treatments with proven efficacy through the dosing interval are prescribed to maintain T below castration levels.