Background: Androgen receptor splice variant (AR-V) expression has previously been regarded as a negative predictive biomarker for response to abiraterone and enzalutamide in mCRPC patients. However, recent data questions this association. We designed a whole blood assay to detect AR-V7 and AR-V9, the two most abundantly expressed AR-Vs, and correlated expression with clinical outcomes in patients commencing abiraterone or enzalutamide. Methods: We developed a quantitative real-time polymerase chain reaction assay to detect AR-V7 and AR-V9 from whole blood collected in PAXgene tubes. The assay was applied to samples prospectively collected from 37 mCRPC patients prior to commencing abiraterone or enzalutamide, and at treatment cessation. Patients positive for either AR-V7 or AR-V9 were defined as AR-V-positive, and AR-V-negative if neither variant was detected. AR-V expression was correlated with PSA response rate (chi-square test) and PSA progression-free survival (PSA-PFS) (log-rank test). Assay sensitivity was determined by serially diluting RNA from VCaP prostate cancer cells (known to express AR-V7) to establish a lower limit of detection. Results: The median follow-up was 7.29 months (IQR 4.21-10.55); 9 of 37 patients (24%) were AR-V-positive. We observed similar response rates in AR-V-positive (6/9) and AR-V-negative (18/28) patients (66% vs. 64%, p = 0.896). PSA-PFS did not differ significantly between groups (9.2 months vs. not reached, p = 0.355). Two patients converted from AR-V-negative to AR-V-positive (PSA-PFS 3.35 and 0.60 months respectively), and one patient remained AR-V-positive at baseline and end-of-treatment sampling. The lower limit of detection for AR-V7 was 0.1%, and AR-V7/V9 was not detected in any of the 13 normal male controls. Conclusions: We developed a sensitive and specific whole blood assay for AR-V7 and AR-V9 detection in patients with mCRPC. Neither PSA response rates nor PSA-PFS differed significantly between AR-V-positive and AR-V-negative patients. These data support recent literature questioning the role of AR-V expression as a negative predictive biomarker in mCRPC patients receiving abiraterone or enzalutamide.