PurposeWe designed this study to identify the prognostic value of baseline prognostic nutritional index (PNI) in metastatic castration-resistant prostate cancer (mCRPC) patients treated with abiraterone acetate or enzalutamide. MethodsOne hundred one mCRPC patients were included. PNI was calculated using formula 10 × serum albumin value (g/dl)+.005 × total lymphocyte count (per mm3). ROC analysis was used for determining prognostic PNI value. ResultsThe statistically significant cut-off value for PNI was 46.62. Initial PSA response and PSA kinetics (early PSA response and 30-50%-90% PSA response at any time) were much better in PNI>46.62 group than the PNI ≤46.62 group (P<.01). In multivariate analysis, baseline PNI level >46.62 was an independent predictor of PSA-PFS (HR: .42; P<.01), radiologic PFS (HR: .53; P<.01), and OS (HR: .42; P<.01). In the PNI≤46.62 group, median OS was 7.4 months (95% CI: 4.1-10.7) for the abiraterone acetate subgroup vs. 17.6 months (95% CI: 10.1-25.1) for enzalutamide subgroups (P<.01). ConclusionPNI is a useful, independent prognostic marker for mCRPC patients treated with either abiraterone acetate or enzalutamide. Using pre-treatment PNI may help clinicians in the prediction of survival and decision making based on abiraterone acetate or enzalutamide.