This study investigates the association between the presence of androgen-receptor splice variant 7 (AR-V7) in circulating tumor cells and resistance to enzalutamide and abiraterone in patients with castration-resistant prostate cancer. AR-V7, which lacks the ligand-binding domain of the androgen receptor, remains constitutively active as a transcription factor. The study enrolled 62 patients with metastatic castration-resistant prostate cancer who were initiating treatment with either enzalutamide or abiraterone. AR-V7 was detected in circulating tumor cells in 39% of enzalutamide-treated patients and 19% of abiraterone-treated patients. Patients with detectable AR-V7 had lower rates of prostate-specific antigen (PSA) response, shorter PSA progression-free survival, clinical or radiographic progression-free survival, and overall survival compared to those without detectable AR-V7. The association between AR-V7 detection and therapeutic resistance was maintained after adjustment for the expression of full-length androgen receptor mRNA. These findings suggest that AR-V7 detection in circulating tumor cells may be a biomarker for resistance to enzalutamide and abiraterone, but further large-scale prospective validation is needed.This study investigates the association between the presence of androgen-receptor splice variant 7 (AR-V7) in circulating tumor cells and resistance to enzalutamide and abiraterone in patients with castration-resistant prostate cancer. AR-V7, which lacks the ligand-binding domain of the androgen receptor, remains constitutively active as a transcription factor. The study enrolled 62 patients with metastatic castration-resistant prostate cancer who were initiating treatment with either enzalutamide or abiraterone. AR-V7 was detected in circulating tumor cells in 39% of enzalutamide-treated patients and 19% of abiraterone-treated patients. Patients with detectable AR-V7 had lower rates of prostate-specific antigen (PSA) response, shorter PSA progression-free survival, clinical or radiographic progression-free survival, and overall survival compared to those without detectable AR-V7. The association between AR-V7 detection and therapeutic resistance was maintained after adjustment for the expression of full-length androgen receptor mRNA. These findings suggest that AR-V7 detection in circulating tumor cells may be a biomarker for resistance to enzalutamide and abiraterone, but further large-scale prospective validation is needed.