Background: It has been shown that HOXB13 is an important androgen receptor (AR) coregulator. The primary objective of this analysis was to correlate HOXB13 expression with clinical outcomes in men with mCRPC. Methods: We conducted a retrospective analysis of the multicenter prospective PROPHECY trial of mCRPC men (n=118) treated abiraterone (abi) or enzalutamide (enza). CTC detection and specific HOXB13 cDNA detection and expression was measured using a modified Adnatest using appropriate controls. CTC HOXB13 status was categorized into 3 groups: undetectable CTCs (CTC=0 based on CTC PSMA & PSA=0); CTC+ but HOXB13 CTC negative (<4 copies) or positive (≥4 copies) where the HOXB13 cutoff point was determined by the maximally selected rank statistic for prognostic associations. The association between HOXB13 and overall survival (OS) and radiographic progression-free survival (rPFS) were explored using the proportional hazards model. We also explored the association between the two endpoints and multigroup of HOXB13 at cutoff 0 and CellSearch CTC at cutoff 1 and 5. Results: 102 men who had sufficient CTC cDNA sample for HOXB13 detection. Of those men with detectable CTCs, 19% (n=19) had no detectable HOXB13, while the majority of men at 67% (n=58) had both CTC and HOXB13 detection. The proportion of patients with CTC=0, CTC+ HOXB13-, and CTC+ HOXB13+ using the optimal HOXB13 threshold were 25%, 31% and 44%, respectively. Men with HOXB13- CTCs were more likely to have low PSA (median 3 vs 79 copies), low CTC PSMA (0 vs 99 copies), low AR-FL (47 vs. 89 copies) and CTC AR-V7 (5.9 vs. 42 copies), and more likely to have liver/lung metastases (41% vs 25%). Median OS were 25.7, 25 and 12.1 months while the median rPFS of the three groups were 9.0, 7.2 and 4.7 months, respectively. Compared to CTC=0, men with HOXB13- CTCs did not have a worse hazard rate for OS (HR: 0.9, 95% CI: 0.4-1.9), while HOXB13+ CTC patients had a worse hazard for OS (HR: 2.0, 95% CI: 1.0-3.9), adjusting for prior abi/enza treatment and Halabi clinical risk score. The positive HOXB13 had also statistically significant increased hazard for progression/death (HR: 2.31, 95% CI: 1.24-4.30). Compared to the group with both HOXB13=0 and CellSearch CTC=0, men with HOXB13>0 and CellSearch CTC≥5 had increased hazard on both OS and rPFS (OS: HR:2.39, 95% CI: 1.06-5.4; rPFS: HR: 2.78, 95% CI: 1.38-5.59). Conclusions: Higher CTC HOXB13 expression is associated with AR dependent biomarkers in CTCs and is adversely prognostic in the context of potent AR inhibition in men with mCRPC, and this assay could be useful to select for patients appropriate for AR or future HOXB13 targeted therapies. Clinical trial information: NCT02269982.