Purpose: Beyond enumeration, circulating tumor cells (CTCs) can provide genetic information from metastatic cancer that may facilitate a greater understanding of tumor biology and enable a precision medicine approach.Experimental Design: CTCs and paired leukocytes from men with metastatic castration-resistant prostate cancer (mCRPC) were isolated from blood through red cell lysis, CD45 depletion, and flow sorting based on EpCAM/CD45 expression. We next performed whole genomic copy number analysis of CTCs and matched patient leukocytes (germline) using array-based comparative genomic hybridization (aCGH) from 16 men with mCRPC, including longitudinal and sequential aCGH analyses of CTCs in the context of enzalutamide therapy.Results: All patients had mCRPC and primary or acquired resistance to abiraterone acetate or enzalutamide. We compiled copy gains and losses, with a particular focus on those genes highly implicated in mCRPC progression and previously validated as being aberrant in metastatic tissue samples and genomic studies of reference mCRPC datasets. Genomic gains in >25% of CTCs were observed in AR, FOXA1, ABL1, MET, ERG, CDK12, BRD4, and ZFHX3, while common genomic losses involved PTEN, ZFHX3, PDE4DIP, RAF1, and GATA2 Analysis of aCGH in a sample with sequential enzalutamide-resistant visceral progression showed acquired loss of AR amplification concurrent with gain of MYCN, consistent with evolution toward a neuroendocrine-like, AR-independent clone.Conclusions: Genomic analysis of pooled CTCs in men with mCRPC suggests a reproducible, but highly complex molecular profile that includes common aberrations in AR, ERG, c-MET, and PI3K signaling during mCRPC progression, which may be useful for predictive biomarker development. Clin Cancer Res; 23(5); 1346-57. ©2016 AACR.