OBJECTIVE Reciprocal plasticity exists between endothelial and mesenchymal lineages. For instance, mature endothelial cells adopt a smooth muscle-like phenotype through transforming growth factor beta-1 (TGFbeta1)-driven endothelial-to-mesenchymal transdifferentiation (EndMT). Peripheral blood contains circulating endothelial progenitor cells of which the endothelial colony-forming cells (ECFCs) harbour stem cell-like properties. Given the plasticity between endothelial and mesenchymal lineages and the stem cell-like properties of ECFCs, we hypothesized that ECFCs can give rise to smooth muscle-like progeny. RESULTS ECFCs were stimulated with TGFbeta1, after which TGFbeta signalling cascades and their downstream effects were investigated. Indeed, EndMT of ECFCs resulted in smooth muscle-like progeniture. TGFbeta1-driven EndMT is mediated by ALK5 kinase activity, increased downstream Smad2 signalling, and reduced protein levels of inhibitor of DNA-binding protein 3. ECFCs lost expression of endothelial markers and endothelial anti-thrombogenic function. Simultaneously, mesenchymal marker expression was gained, cytoskeletal rearrangements occurred, and cells acquired a contractile phenotype. Transdifferentiated ECFCs were phenotypically stable and self-sustaining and, importantly, showed fibroblast growth factor-2 and angiopoietin-1-mediated pro-angiogenic paracrine properties. CONCLUSIONS Our study is the first to demonstrate that ECFCs can give rise to smooth muscle-like progeny, with potential therapeutic benefits. These findings further illustrate that ECFCs are highly plastic, which by itself has implications for therapeutical use.