Background: The androgen receptor splice variant 7 (AR-V7) is associated with resistance to hormonal therapy in castration-resistant prostate cancer (CRPC). Due to limitations of the methods available for AR-V7 analysis, the identification of a reliable detection method may facilitate the use of this biomarker in clinical practice. Objective: To confirm AR-V7 as a predictor of resistance to hormonal therapy and develop a new approach to assess AR-V7 by highly sensitive digital droplet polymerase chain reaction (ddPCR) in plasma-derived exosomal RNA. Design, setting, and participants: Plasma samples were collected from 36 CRPC patients before they began second-line hormonal treatment. Exosomes were isolated and RNA extracted for analysis of AR-V7 by ddPCR. Outcome measurements and statistical analysis: The absolute target gene concentration as copies per milliliter (copies/ml) was determined by ddPCR. Statistical analyses were performed with SPSS software (IBM Corp., Armonk, NY, USA). Results and limitations: A total of 26 patients received abiraterone and 10 enzalutamide; 39% of patients were found to be AR-V7 positive (AR-V7(+)). Median progression-free survival was significantly longer in AR-V7 negative (AR-V7(-)) versus AR-V7(+) patients (20 vs 3 mo; p < 0.001). Overall survival was significantly shorter in AR-V7(+) participants at baseline compared with AR-V7(-) participants (8 mo vs not reached; p < 0.001). Conclusions: This study demonstrates that plasma-derived exosomal RNA is a reliable source of AR-V7 that can be detected sensitively by ddPCR assay. We also showed that resistance to hormonal therapy may be predicted by AR-V7, making it a clinically relevant biomarker. Patient summary: We report a first study on a method for androgen receptor splice variant 7 (AR-V7) detection in RNA extracted from cancer cell vesicles released in blood. Results confirmed the role of AR-V7 as a predictive biomarker of resistance to hormonal therapy. Our assay showed that vesicles are a reliable source of AR-V7 RNA and that the method is fast, highly sensitive, and affordable. (C) 2016 European Association of Urology. Published by Elsevier B.V. All rights reserved.