Background: Coronary artery bypass grafting (CABG) surgery continues to be the gold standard for treating the patients with coronary artery disease. CABG surgery can be performed on or off cardiopulmonary bypass, termed as on- pump or off-pump CABG, respectively. It has been shown that CABG surgery, preferably on-pump CABG surgery, leads to the changes of cell immunity during perioperative and early postoperative period. The mechanisms of regulation of the immune response in patients during and early after surgical revascularization are not fully understood. The aim of this study was to investigate the influence of carbohydrate preoperative oral feeding on frequency and perforin expression in peripheral blood lymphocytes in patients after on- or off-pump CABG surgery in early postoperative period. Patients and methods: In this prospective clinical study, 80 patients scheduled for CABG surgery were included in the study. The patients were randomly allocated into four groups (20 in each group): patients in Group 1 underwent on- pump CABG and did not receive carbohydrate preoperative oral feeding ; patients in Group 2 underwent on-pump CABG and were preoperatively fed ; patients in Group 3 underwent off- pump CABG and did not receive carbohydrate preoperative oral feeding ; while patients in Group 4 underwent off-pump CABG and received carbohydrate preoperative oral feeding. Blood samples were collected immediately before (T1), 24 (T2) and 72 (T3) hours after the surgery. Peripheral blood mononuclear cells were isolated by gradient centrifugation and simultaneously labelled by antigens using fluorochrome- conjugated monoclonal antibodies. Frequency of T lymphocytes, NK and NKT cells, their subsets as well as their perforin expression were detected, and analyzed by flow cytometry. Results: There was significant decrease in frequency of CD3+ and CD3+CD4+ cells, as well as perforin expressing CD3+CD8+ cells in patients who underwent on-pump CABG in comparison to patients who underwent off- pump CABG 24 hours after the surgery. Carbohydrate preoperative oral feeding did not effect changes in lymphocytes subpopulations and perforin expression at any time point. Conclusion: Decreases of CD3+ cells on account of CD3+CD4+ subsets, and perforin expressing cells on account of CD3+CD8+ perforin+ cells were found in patients who had undergone on-pump CABG, but not in patients who had undergone off-pump CABG surgery, irrespectively of carbohydrate preoperative oral feeding.