BACKGROUND The therapeutic potential of endothelial colony-forming cells (ECFCs) may be impaired in an ischemic environment. Direct injection of ECFCs is not an effective method of rescuing the ischemic heart, but exosomes derived from these cells may be a promising therapeutic tool. However, exosomes produced under normoxia and hypoxia may not be identical. Therefore, the purpose of this study was to investigate alterations in the anti-fibrotic effects of hypoxia-treated ECFC-derived exosomes and the underlying mechanism involved. MATERIAL AND METHODS ECFCs were isolated from peripheral blood and exosomes were collected from ECFCs treated with normoxia (nor-exo) or hypoxia (hyp-exo). Effects of exosomes on cardiac fibroblast activation were evaluated in vitro. MicroRNAs (miRNAs) inside the exosomes were extracted and compared using next-generation RNA sequencing. Predicted target mRNAs of miR-10b-5p were validated using a dual-luciferase reporter gene assay method. RESULTS Nor-exo significantly ameliorated cardiac fibroblast activation in vitro. These effects were attenuated in the hyp-exo-treated group. miR-10b-5p was enriched in nor-exo but not in hyp-exo. Dual-luciferase reporter gene assay found that both SMAD-specific E3 ubiquitin protein ligase 1 (Smurf1) and histone deacetylase 4 (HDAC4) mRNAs were inhibited by miR-10b-5p. The expression of neutral sphingomyelinase 2 (N-SMase2) was decreased in hypoxia ECFCs, and this result was consistent with the changes in miR-10b-5p in hyp-exo. CONCLUSIONS Due to a reduction of miR-10b-5p, which targets the fibrotic genes Smurf1 and HDAC4, the anti-fibrotic effects of hyp-exo were abolished.