We aimed to study the number of endothelial progenitor cells (EPCs) in atherosclerotic patients who underwent bypass grafting, or valve replacement. Plasma from these patients was added to cultured EPCs from healthy blood donors in order to try to reproduce in vitro what found in vivo. EPC number was assessed by flow cytometry, as percentage of CD34⁺/CD144⁺/CD3^–, CD34⁺/KDR⁺/CD3^– and CD14⁺/CD105⁺/CD3^– mononuclear cells. 44 bypass patients and 43 valvular patients were chosen. For EPC cultures, mononuclear cells were obtained from healthy blood donors and separated using ficoll. Cells were plated on fibronectin and incubated with MV2 medium. Apoptosis was assessed by DNA fragmentation. Protein expression was measured by Western blot. The study was conducted according to the Declaration of Helsinki (revised in 2000). The hospital ethic committee approved the protocol and we obtained informed consent from all subjects before sampling took place. Bypass patients had lower numbers of CD34⁺/CD144⁺ EPCs, compared to valvular (0.2239±0.032% vs 0.4370±0.08%, p=0.0187), and lower numbers of CD34⁺/CD144⁺/CD3^– EPCs (0.3309±0.0.054% vs 0.7786±0.1922%, p=0.026). However, cultured EPCs from healthy donors exhibited a lower apoptotic rate after 24, 48 or 72h incubation with 1% plasma from bypass patients, compared to valvular (0.9998 vs 2.038, 0.9971 vs 1.951 y 0.998 vs 1.599, respectively), being this effect prevented by TGFbeta blockers SIS3 and SB431542. Incubation with 1% plasma from bypass patients induced higher expression of CD105 and CD144, (measured by Western blot in at least two different experiments, using plasma from three different patients each one). Atherosclerotic patients possess a lower pre-surgical number of EPCs, compared to valvular. EPC dysfunction in culture is not observed in EPCs from healthy donors incubated with plasma from atherosclerotic patients. The widely described in vitro EPC dysfunction in atherosclerotic patients may be due to intrinsic defects in these cells from the bone marrow.