Background: PC is characterized by a relatively low prevalence of recurrent somatic point mutations. ctDNA is shed from PC and can be analyzed to profile somatic point mutations and copy number changes. We evaluated a computational approach to detect ctDNA (ie. ctDNA+) in PC based on allele frequencies of polymorphisms and mutations. We then sought to confirm the association of this biomarker with disease burden and clinical outcome. Methods: Customized, hybrid capture, high-depth next-generation sequencing was performed on pre-treatment (PT) plasma samples from a phase 2 line 3+ metastatic castration-resistant PC (mCRPC) study (NCT02854436, GALAHAD) and PT and end of treatment (EOT) samples from randomized Phase 3 study in non-metastatic (nm) CRPC (NCT01946204, SPARTAN) and from metastatic castration-sensitive PC (mCSPC) (NCT02489318, TITAN). Associations of ctDNA+ with bone lesions (number), visceral metastases (+/-), circulating tumor cells count (CTCc), and serum prostate specific antigen (PSA), alkaline phosphatase (AP) and lactate dehydrogenase (LD) were tested. Also, associations of ctDNA+ with overall survival (OS) and second progression free survival (PFS2) were evaluated in randomized studies using Cox regression. Results: ctDNA+ at PT was 7.5% in nmCRPC, 23.7% in mCSPC and 66% in heavily pre-treated mCRPC. ctDNA+ increased from PT to EOT in nmCRPC (7.5% to 27%) and mCSPC (23.7% to 63.6%). Disease burden metrics were evaluated in ctDNA+ vs ctDNA- patients. ctDNA+ was associated with higher disease burden in mCRPC (Table), nmCRPC and mCSPC. At EOT, ctDNA+ patients had shorter OS and PFS2 in nmCRPC (HR [95% CI] OS: 2.73 [1.83, 4.08], p < 0.0001; PFS2: 2.00 [1.38, 2.90], p = 0.0002) and mCSPC (HR [95% CI] OS: 7.59 [3.22, 17.91], p < 0.0001; PFS2: 4.84 [2.47, 9.47], p < 0.0001). Conclusions: ctDNA+ assessed using our novel, composite biomarker increases with advanced disease state and disease progression, is significantly associated with disease burden and poor clinical outcome in PC and could be a clinically relevant metric for monitoring response to therapy. Clinical trial information: NCT02854436.