Background: The potent regenerative capability of the vasculature is attributed to populations of resident progenitors, termed endothelial colony-forming cells (ECFCs). ECFCs maintain endothelial function and is capable neovascularisation in vivo, implicating their role in tissue vascular regeneration. However, ECFCs can often acquire mesenchymal phenotype, via a process termed endothelial-to-mesenchymal transition (EndMT), which is thought to contribute to the development of atherosclerosis. Our study aims to address the pathological catalyst that drives EndMT and delineate its role in the progression of atherosclerosis. Methods and Results: In the presence of excessive oxidised low lipoprotein (ox-LDL), cultured ECFCs demonstrated relative mRNA and protein increase in mesenchymal markers such as Fsp-1, and transcription factors such as Sox9 and Runx2. Long-term treatment of ECFCs with ox-LDL directly contributed to accelerated de-differentiation towards a mesenchymal phenotype and subsequent depletion of the ECFC progenitor pool, observed under flow cytometry. The self-renewal capacity of ECFCs, assessed through single-cell colony formation assay, was also significantly reduced following 5-days of ox-LDL treatment. Conclusion: The finding of this study supports the idea that atherosclerosis is a consequence of endothelial progenitor depletion and fate choice, representing a paradigm shift from the current understanding and open new avenues for preventive therapies.