Background: The natural history of BCR after prostatectomy is variable and current treatment approaches include observation, salvage radiation, or androgen deprivation therapy (ADT), although many men wish to defer such therapy. We hypothesized that olaparib, now approved for use in certain men with metastatic castration resistant prostate cancer, might be an alternative option for men with high-risk BCR. Methods: This was a single arm, phase 2 trial (NCT03047135) conducted at 4 US sites. Subjects with biochemically recurrent (M0) prostate cancer, post prostatectomy, with a PSA doubling time of ≤6 months and an absolute PSA of ≥1.0 ng/mL were eligible. Patients were NOT selected based on the presence of homologous recombination repair (HRR) mutations. 51 patients were treated with olaparib 300mg twice per day, until radiographic or symptomatic progression, PSA doubling from baseline value, or drug-related toxicity. The primary endpoint was PSA50 response (50% PSA decline from baseline). Secondary endpoint was PSA progression free survival (PFS). Patients were required to undergo tissue-based next generation sequencing on the radical prostatectomy specimen to determine the presence of mutations in HRR genes (ATM, BARD1, BRCA1/2, BRIP1, CDK12, CHEK1/2, FANCA/E/L, PALB2, RAD51B/C/D). A prespecified endpoint was to assess PSA responses in HRR[+] and HRR[–] cohorts separately. Results: Mean age was 64 years (SD 7), and 92% (47/51) of patients were White. Mean baseline PSA doubling time was 2.9 (SD 1.5) months. 55% (28/51) of pts had Gleason 6-7, 14% (7/51) had Gleason 8, and 31% (16/51) had Gleason 9-10. 86% (44/51) of patients had previously received salvage radiation. 27 patients harbored HRR mutations, the most common of which were BRCA2 (n=11), CHEK2 (n=6), and ATM (n=6). The overall PSA50 response rate was 24% (12 of 51)(95% CI 0.13, 0.38), with no responses in the HRR[–] group and 44% (12/27) PSA50 responses in the HRR[+] group (95%CI 0.26, 0.65). All 11 of the participants with a BRCA2 mutation achieved a PSA50 response. The median PSA PFS was 22 months in the HRR[+] group and 13 months in the HRR[–] group (HR 0.71; 95% CI 0.31, 1.63). Most AEs were low grade. The most common all-grade AEs were fatigue, nausea, leukopenia, anemia, and dysgeusia. Grade-3 AEs were leukopenia (9.8%), ALT/AST increase (3.9%), and anemia (2.0%). There were no grade 4–5 AEs. Conclusions: Olaparib, in the absence of ADT, can be safely administered and demonstrated activity in men with BCR prostate cancer and HRR mutations, especially for BRCA2 mutations. Additional studies to further evaluate olaparib in these patients are indicated. Clinical trial information: NCT03047135.