Background: Expression of androgen receptor splice variant 7 (AR-V7) in circulating tumor cells (CTCs) has been associated with resistance to ABI and enzalutamide (ENZ) (Antonarakis et al. N Engl J Med 2014). We have developed a RT-PCR assay that is neither time sensitive nor requires CTC enrichment for detection of prostate cancer (PCa)-associated transcripts in whole blood. Using this assay, our aim was to correlate AR-V7 expression with outcomes on ABI. Methods: 2.5 ml whole blood was collected into PAXgene RNA tubes from mCRPC patients (pts) commencing ABI. RT-PCR was performed for the following genes: AR-V7, FOXA1, GRHL2, HOXB13, KLK2, KLK3 and TMPRSS2:ERG. For each gene, the highest CT value among 20 normal controls was set as the threshold for a positive (+) test. Clinical endpoints were PSA50 response rate (RR) (PSA decline ≥ 50% confirmed ≥ 3 weeks later), PSA30 RR, time to PSA (PCWG2 criteria) or clinical progression (change in anti-cancer therapy or decline in ECOG performance status (PS) ≥ 2 levels due to PCa), and overall survival (OS). Results: Whole blood samples were obtained from 37 pts. Median age was 70. 59% received prior docetaxel. All pts were ABI and ENZ naïve. PSA50 RR was 37% and PSA30 RR was 48%. Median progression-free survival (PFS) was 3.8 months (mos) and median OS was 21.0 mos. 11% (4/37) of pts were AR-V7+. AR-V7+ pts were more likely to have high ALP (P= 0.04; Χ²), high LDH (P= 0.07) and ECOG PS ≥ 2 (P= 0.052). Pts with an AR-V7+ test had lower PSA50 RR (0% vs. 42%, P= 0.10; Χ²) and PSA30 RR (0% vs. 52%, P= 0.051) together with shorter median PFS (0.7 mos vs. 4.0 mos, P< 0.001; log-rank) and median OS (5.5 mos vs. 22.1 mos, P< 0.001). Other factors linked with worse OS were high ALP (P= 0.02), liver metastases (P= 0.03), ≤ 36 mos on primary hormone therapy (P= 0.04), HOXB13+ (P= 0.03) and KLK2+ (P< 0.001). Conclusions: RT-PCR detection of AR-V7 transcripts in whole blood was associated with a 0% PSA RR and significantly inferior PFS and OS in pts treated with ABI. These results reinforce the potential utility of AR-V7 as a prognostic and predictive biomarker for mCRPC. Validation of the assay in larger datasets is ongoing.