Recently, some studies have shown that nicotine increased neovascularization, which involves endothelial progenitor cells (EPCs). The effects of nicotine on EPCs are still unclear at present. Therefore, the authors investigated whether nicotine had influences on EPC number and activity. The EPCs were stimulated with nicotine (to make a series of final concentrations: 10(-12) mol/L, 10(-10) mol/L, 10(-8) mol/L, 10(-6) mol/L, 10(-4) mol/L) or vehicle control for the respective time points(12, 18, 24, 32, and 48 hours). The EPCs were characterized as adherent cells double positive for DiLDL uptake and lectin binding by direct fluorescent staining under a laser-scanning confocal microscope. They were further documented by demonstrating the expression of KDR, VEGFR-2, and AC133 with flow cytometry. The EPC proliferation, migration, and in vitro vasculogenesis activity were assayed with the 3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide assay; the modified Boyden chamber assay; and the in vitro vasculogenesis kit, respectively. The EPC adhesion assay was performed by replating those on fibronectin-coated dishes and then counting the adherent cells. As a result, nicotine dose dependently increased the EPC number and the proliferative, migratory, adhesive, and in vitro vasculogenesis capacity at nicotine concentrations of 10(-12) to 10(-8) mol/L. The peak effects on EPCs were observed at concentrations of nicotine 10(-8) mol/L, similar to those in the blood of smokers. In addition, nicotine (10(-8) mol/L) time dependently increased the EPC number and activity. However, cytotoxicity was seen at higher nicotine concentrations (> 10(-6) mol/L). In conclusion, nicotine had complex effects on EPCs: nicotine might induce the augmentation of EPCs with enhanced functional activity at relatively low concentrations. However, cytotoxicity was seen at higher nicotine concentrations.