OBJECTIVE: We compared profiles of the numbers of circulating endothelial cells (CEC) and the apoptosis-inducing capacity of serum samples on human endothelial cells (hEC) in on-pump and off-pump coronary artery bypass grafting (CABG) patients. METHODS: Blood samples from 30 patients undergoing CABG (randomly assigned to two groups: 15 patients off-pump and 15 on-pump (cardiopulmonary bypass, CPB)) were collected after induction of anesthesia (preoperatively), at weaning from CPB/end of bypass grafting (0 h), and 1, 6, 12, 24, and 48 h afterwards. CEC were isolated with immunomagnetic anti-CD146-coated Dynabeads, and counted in a Nageotte chamber. The apoptosis-inducing activity of serum samples on hEC was examined by a tissue culture assay system. Apoptotic and normal cells were identified using phase contrast/fluorescence microscopy after DNA dye staining. RESULTS: CEC numbers and proportions of apoptotic hEC were significantly elevated during and after surgery in both groups (p<0.01). Compared with the on-pump group, CEC and proportions of apoptotic hEC were significantly lower (p=0.04 and p=0.03, respectively) in patients having CABG performed off-pump. Starting at comparable baseline levels, the mean CEC-number was highest at 6h postoperatively with 81.9 ml(-1) (range, 44-141) for on-pump patients and 63.3 ml(-1) (range, 48-105) for off-pump patients. hEC apoptosis peaked also at T4: 16.5+/-2.8% versus 11.3+/-2.2%. In both groups, CEC numbers and proportions of endothelial apoptosis were still elevated at 48 h after surgery. CONCLUSION: The number of circulating endothelial cells and apoptotic endothelial cell death are markers of endothelial activation and damage during CABG. This study provides evidence that CABG with the use of CPB in comparison to OPCAB surgery is associated with a significantly more pronounced endothelial response in the immediate postoperative period.