Purpose: Translational studies have shown that CDK12 mutations may delineate an immunoresponsive subgroup of prostate cancer, characterized by high neo-antigen burden. Given that these mutations may define a clinically distinct subgroup, we sought to describe outcomes to standard drugs and checkpoint inhibitors (CPI)., Patients and Methods: Clinical data from consecutive patients with CDK12 mutations were retrospectively collected from 7 centers. Several clinical-grade sequencing assays were used to assess CDK12 status. Descriptive statistics included PSA50 response rate (≥ 50% decline in prostate-specific antigen from baseline) and clinical/radiographic progression-free survival (PFS)., Results: Of 52 patients with CDK12 -mutated prostate cancer, 27 (52%) had detected biallelic CDK12 alterations. At diagnosis, 44 (88%) had Gleason grade group 4-5, 52% had T3-T4, and 14 (27%) had M1 disease. Median follow-up was 8.2 years (95% CI, 5.6 to 11.1 years), and 49 (94%) developed metastatic disease. Median overall survival from metastasis was 3.9 years (95% CI, 3.2 to 8.1 years). Unconfirmed PSA50 response rates to abiraterone and enzalutamide in the first-line castration-resistant prostate cancer setting were 11 of 17 (65%) and 9 of 12 (75%), respectively. Median PFS on first-line abiraterone and enzalutamide was short, at 8.2 months (95% CI, 6.6 to 12.6 months) and 10.6 months (95% CI, 10.2 months to not reached), respectively. Nineteen patients received CPI therapy. PSA50 responses to CPI were noted in 11%, and PFS was short; however, the estimated 9-month PFS was 23%. PFS was higher in chemotherapy-näıve versus chemotherapypretreated patients (median PFS: not reached v 2.1 months, P = .004)., Conclusion: CDK12 mutations define an aggressive prostate cancer subgroup, with a high rate of metastases and short overall survival. CPI may be effective in a minority of these patients, and exploratory analysis supports using anti-programmed cell death protein 1 drugs early. Prospective studies testing CPI in this subset of patients with prostate cancer are warranted., Competing Interests: AUTHORS’ DISCLOSURES OF POTENTIAL CONFLICTS OF INTEREST The following represents disclosure information provided by authors of this manuscript. All relationships are considered compensated unless otherwise noted. Relationships are self-held unless noted. I = Immediate Family Member, Inst = My Institution. Relationships may not relate to the subject matter of this manuscript. For more information about ASCO’s conflict of interest policy, please refer to www.asco.org/rwc or ascopubs.org/po/author-center. Open Payments is a public database containing information reported by companies about payments made to US-licensed physicians (Open Payments). Michael T. Schweizer Consulting or Advisory Role: Janssen Research Funding: Janssen (Inst), AstraZeneca (Inst), Roche (Inst), Pfizer (Inst), Zenith Epigenetics (Inst), Madison Vaccines (Inst), Immunomedics (Inst) Gavin Ha Patents, Royalties, Other Intellectual Property: Methods for genome characterization (US20190078232A1) patent application status is pending Rana R. McKay Consulting or Advisory Role: Janssen, Novartis, Tempus, Exelixis, Pfizer, Bristol-Myers Squibb, Astellas Medivation, Dendreon Research Funding: Pfizer (Inst), Bayer (Inst) Tanya Dorff Consulting or Advisory Role: Bayer, Janssen Oncology, AstraZeneca, Roche, Seattle Genetics, Noxopharm, Bristol-Myers Squibb Speakers’ Bureau: Exelixis, Prometheus Laboratories. Research Funding: Bristol-Myers Squibb, Bayer Deepak Kilari Honoraria: Exelixis Consulting or Advisory Role: Exelixis, Sanofi Speakers’ Bureau: Janssen, Exelixis, Genzyme (Inst), Genzyme Research Funding: Astellas Pharma (Inst) Travel, Accommodations, Expenses: Sanofi, Exelixis, Janssen, Bayer, Clovis Oncology William K. Oh Leadership: CheckPoint Sciences Stock and Other Ownership Interests: Bellicum Pharmaceuticals Consulting or Advisory Role: Sanofi, Janssen, AstraZeneca, Amgen, Bayer, TYME, CheckPoint Sciences, Sema4, Huya Biosciences, TeneoBio Research Funding: Sotio (Inst), Constellation Pharmaceuticals Arul Chinnaiyan Stock and Other Ownership Inteests: Oncopia, Tempus, Esanik, OncoFusion Therapeutics, Medsyn Consulting or Advisory Role: Tempus Patents, Royalties, Other Intellectual Property: University of Michigan royalties Colin C. Pritchard Consulting or Advisory Role: Promega Andrew J. Armstrong Honoraria: Dendreon, Janssen Oncology Consulting or Advisory Role: Bayer, Sanofi, Dendreon, Medivation, Janssen Biotech, Pfizer, Astellas Scientific and Medical Affairs, Clovis Oncology, AstraZeneca Speakers’ Bureau: Dendreon, Bayer Research Funding: Dendreon (Inst), Sanofi (Inst), Bayer (Inst), Pfizer (Inst), Novartis (Inst), Janssen Oncology (Inst), Medivation (Inst), Astellas Pharma (Inst), Gilead Sciences (Inst), Roche/Genentech (Inst), Active Biotech (Inst), Bristol-Myers Squibb (Inst), Constellation Pharmaceuticals (Inst), Merck (Inst) Patents, Royalties, Other Intellectual Property: Circulating tumor cell novel capture technology (Inst) Travel, Accommodations, Expenses: Dendreon, Janssen Biotech, Bayer, Astellas Scientific and Medical Affairs R. Bruce Montgomery Research Funding: AstraZeneca (Inst), Janssen Oncology (Inst), Clovis (Inst), Astellas Pharma (Inst), Beigene (Inst) Ajjai Alva Consulting or Advisory Role: AstraZeneca, Merck, Pfizer, Bristol-Myers Squibb Speakers’ Bureau: AstraZeneca Research Funding: Genentech (Inst), Bristol-Myers Squibb (Inst), Merck Sharp & Dohme (Inst), Prometheus Laboratories (Inst), Mirati Therapeutics (Inst), AstraZeneca (Inst), Roche (Inst), Bayer (Inst), Progenics (Inst), Astellas Pharma (Inst), Arcus Biosciences (Inst), Harpoon Therapeutics (Inst), Progenics (Inst), Celgene (Inst), Janssen (Inst) Travel, Accommodations, Expenses: Merck, BMS No other potential conflicts of interest were reported.