Objective: Endothelial progenitor colony forming units (CFU-ECs) were first believed to be the progenitors of endothelial cells and were named as endothelial progenitor cells (EPCs). But further it was discovered that they are monocytes regulating vasculogenesis. CFU-derived cells have been shown to improve blood flow recovery and capillary density in animal models of hind-limb or myocardial ischemia. The problem of working with these cells for therapeutic purposes is their low frequency and limited replicative potentials. The aim of this study was to determine telomerase activity and alternative splicing variants in CFU-ECs as a potential cause of limited replicative capacity in these cells. Materials and Methods: CFU-ECs were isolated using a standard cell culture assay developed by Hill et al. Colonies were detached mechanically and alternative splicing variant mRNAs were evaluated with real time PCR. Telomerase enzyme activity was assessed using telomerase repeat amplification protocol (TRAP). The same procedures were done on the cancer cell line Calu6 as the positive control. Results: Cells showed excellent colonies during culture improving proper isolation of CFU-ECs. Telomere length amplification protocol (TRAP) assay showed no telomerase activity and Real Time polymerase chain reactions (PCR) showed no telomerase enzyme mRNA but both were significantly high in the cancer cell line. Conclusion: CFU-ECs do not exhibit telomerase activity and this might be one of the reasons for their limited replicative potential.