BACKGROUND: Colorectal cancer is among the most prevalent causes of tumor-related deaths worldwide. Antiangiogenic therapy represents a cornerstone of metastatic CRC treatment, and biomarkers are advocated for the optimization of this therapeutic strategy. METHODS: In this observational prospective study, we employed an optimized flow cytometry protocol to investigate the prognostic and predictive potential of blood circulating endothelial cells (CECs), circulating endothelial progenitor cells (CEPCs), and related subsets in a cohort of patients with metastatic colorectal cancer ( RESULTS: Computational FC analysis revealed a differential enrichment of blood cell clusters with a CD34+/CD45dim/CD117(c-kit)+ phenotype between responders and non-responders both to antiangiogenic and non-antiangiogenic treatments. Intriguingly, our results show that a high percentage of annexin V-negative cells in a putative circulating progenitor population with a CD34+/CD45dim/CD117+ phenotype was correlated with a reduced response to systemic anticancer treatments ( CONCLUSIONS: Overall, these findings hold promise for the identification of novel circulating biomarkers to develop more personalized treatment approaches in patients with metastatic colorectal cancer.