Background: NEPC, de novo or treatment-related in late stage CRPC, is a distinct entity with poor prognosis. Developing non-invasive methods for detection of NEPC is important for clinical practice and trial enrollment. We previously reported on the clinical and genomic characterization of NEPC (Conteduca et al ESMO 2018). A separate study (Aggarwal et al JCO 2018) suggested that low levels of NSE and CgA were associated with a strong NPV for NEPC on biopsy (bx). This study aimed to validate the utility of NSE and CgA in evaluation of NEPC by comparison with met bx. Methods: Our IRB-approved NEPC database was screened for pts who underwent met bx and had concurrent serum NSE, CgA. Clinical data, serum PSA, LDH, ALP, Hb were recorded at time of bx. Comparison of continuous variables between CRPC adeno and NEPC was assessed by nonparametric Kruskal-Wallis test. ROC curve analysis was performed for evaluation of predictive models with serum NE markers. Results: 152 men were identified, median age 71 yrs (49-97). 35 pts had pure/mixed NEPC, while the rest (N=117) had typical adenoca on bx. Half of pts (80/152, 52.6%) received abiraterone or/and enzalutamide. Liver mets were more common in NEPC pts (P=0.001). Median serum NSE (11.2 vs 8.6 ng/mL, P=0.008) and CgA (211 vs 135 ng/mL, P=0.035) were higher in pts with NEPC vs CRPC adeno (Table). Using ROC curve analysis for NSE (normal 3.7-8.9 ng/mL) and CgA (normal 0-95 ng/mL) as independent diagnostic tests, the following cut-offs were identified: NSE 30.1 (Sn: 37%, Sp: 94%, PPV: 34%, NPV: 82%), CgA 170 (Sn: 63%, Sp: 59%, PPV: 23%, NPV: 83%). Conclusions: Our study confirms the potential utility of serum NSE and CgA in excluding a morphological dx of NEPC when below certain thresholds. However, our findings cannot support deferring a met bx in such cases. Larger studies are needed to evaluate for a more robust predictive ability of serum NE markers.