Introduction: Recently, a prognostic index including six risk factors (RFs) (unfavourable ECOG performance status [PS], presence of liver metastases, short response to luteinizing hormone-releasing hormone [LHRH] agonists/antagonists, low albumin, increased alkaline phosphatase [ALP] and lactate dehydrogenase [LDH]) was developed from the COU-AA-301 trial in post-chemotherapy metastatic castration-resistant prostate cancer (mCRPC) patients treated with abiraterone acetate. Our primary objective was to evaluate this model in a cohort of chemotherapy-naive mCRPC patients receiving abiraterone. Methods: We identified 197 chemotherapy-naive patients who received abiraterone at six BC Cancer Agency centres and who had complete information on all six RFs. Study endpoints were prostate-specific antigen (PSA) response rate (RR), time to PSA progression, time on treatment, and overall survival (OS). PSA RR and survival outcomes were compared using ?2 test and log-rank test. Multivariable Cox proportional hazard analysis was performed to identify RFs independently associated with OS. Results: Patients were classified into good (0?1 RFs), intermediate (2?3 RFs), and poor (4?6 RFs) prognostic groups (33%, 52%, and 15%, respectively). For good, intermediate, and poor risk patients, PSA RR (≥50% decline) was 60% vs. 42% vs. 40% (p=0.05); median time to PSA progression was 7.3 vs. 5.3 vs. 5.0 months (p=0.02); and median OS was 29.4 vs. 13.8 vs. 8.7 months (p<0.0001). Conclusions: The six-factor prognostic index model stratifies clinical outcomes in chemotherapy-naive mCRPC patients treated with abiraterone. Identifying patients at risk of poor outcomes is important for informing clinical practice and clinical trial design.