Although extensive work has been conducted to verify if EPCs impairment plays a key role in coronary atherogenesis, it is still matter of debate if the extension and severity of coronary artery disease are associated with reduced or increased numbers of EPCs, as it remains unclear if these cells exert favorable or unfavorable effects at sites of percutaneous coronary intervention (PCI). One should consider, however, that most previous investigations have been hampered by discordant definitions of EPCs and by different timing of EPCs sampling, thus determining much uncertainty on the role of EPCs in restenosis and atherosclerosis progression. Furthermore, development of de novo lesions and post-PCI restenosis, which are pathophysiologically dissimilar, have not been examined concomitantly and serially over time.