Background: Optimal utilization of novel therapies for advanced prostate cancer is challenging without a validated surrogate efficacy endpoint. Ongoing trials are using durable undetectable prostate specific antigen (PSA) levels as a marker of efficacy. The proportion of patients and clinical relevance of those with a prolonged undetectable PSA after a short course of androgen deprivation therapy (ADT) is uncertain. Methods: The University of Washington Caisis database was queried for radical prostatectomy patients who received 6–12 months of ADT after biochemical recurrence (BCR), defined as PSA ≥ 0.2 ng/mL and no radiographically detectable metastasis. Proportions of patients with undetectable PSA 12 and 24 months after ending ADT were compared to a hypothesized 5% rate using exact binomial tests. Associations with patient and tumor characteristics were examined using logistic regression, and associations with risk of subsequent metastasis and death from any cause were evaluated by log-rank tests. Results: After ineligibility exclusions, data were abstracted from 93 patients. Proportions of patients with undetectable PSA 12 and 24 months after ending ADT were n=23/93 (24.7%; 95% CI 16.4–34.8%; P<0.001) and n=14/93 (15.1%; 95% CI 8.5–24.0%; P<0.001), respectively. Proportions of patients with undetectable PSA 12 and 24 months after testosterone recovery ≥ 50 ng/dL were n=16/65 (24.6%; 95% CI 14.8-36.9%) and n=10/65 (15.4%; 95% CI 7.6-26.5%), respectively. Being 1 year older at diagnosis was associated with an 11.5% (95% CI 3.1–21.9%; P=0.01) increase in the odds of having a detectable PSA after controlling for PSA at diagnosis, Gleason sum and time from initial therapy to BCR. Detectable PSA was associated with increased risk of metastasis (P=0.006) with marginal evidence of association with death from any cause (P=0.07). Conclusions: This single-institution retrospective analysis shows that it is not uncommon to have undetectable PSA 12 or 24 months after a short course of ADT. Additional analysis is needed to demonstrate the clinical value of this measure as a surrogate for prostate cancer outcomes and for consideration as a trial endpoint.