Background: Circulating tumor cell (CTC) counts have clinical utility as a prognostic marker in cancer. For prostate cancer (PCa), CTC counts ≥5 CTC/7.5 ml using the FDA-approved CellSearch system convey a negative prognosis with respect to overall survival, but the significance of changes in CTC count during treatment is not fully understood. To determine whether longitudinal CTC counts can be used as a prognostic and/or predictive marker of clinical outcome in PCa patients undergoing androgen deprivation therapy (ADT) plus chemotherapy, we used the AxonDx nCyte non-enrichment-based CTC detection and enumeration system to evaluate CTC counts at baseline, during, and after therapy in 18 patients for up to 11 months. Methods: Whole blood was drawn from PCa patients on clinical trials for ADT plus chemotherapy at PSA evaluation (±radiographic reevaluation) into vacutainers containing K3-EDTA and processed within 6 hr. PBMC containing nucleated cells harvested from 6 ml of whole blood were fixed, permeabilized and blocked with BSA buffer, and stained using a proprietary antibody cocktail containing DAPI, an anti-cytokeratin mixture, the pan-leukocyte marker CD45, and markers selective for granulocytes and lymphocytes. Stained cells were drawn onto 1.2 µm pore filters, washed, placed on counting slides, and analyzed. CTCs are identified as cytokeratin-positive, nucleus-positive and leukocyte marker-negative. Results: CTCs were obtained from 12 patients for 2-9 months at 2-7 time points; from 2 patients for 11 months at 5-7 time points, and from 4 patients for 1 month at 1 time point. We detected time-dependent changes in CTC counts following treatment that did not necessarily correlate with patient’s local prostate specific antigen (PSA) levels but were early indicators of subsequent increase in tumor burden. Conclusions: The AxonDx nCyte system can reliably detect CTCs from PCa patient samples that trend along with other signs of disease progression. CTCs can be used longitudinally for clinical monitoring of patients with PCa prior to signs of progression via tumor markers or scans during ADT plus chemotherapy, providing a reliable biomarker for PCa.