Changes in the differential blood cell count, monocyte phenotype and the cytokine plasma levels in a group of seven patients with cardiac surgery/cardiopulmonary bypass (CPB) and nine patients with thoracic surgery/without CPB, both receiving identical opioid-based anaesthetic technique, were assessed. A significant reduction in the number of circulating lymphocytes and monocytes was observed after anaesthesia and surgery. Interestingly, at the end of surgery as well as 1 day post-surgery a marked increase in the number of granulocytes was noted. General anaesthesia and surgery caused a significant reduction of HLA-DR and CD11c/CD18 molecules, starting immediately after induction of anaesthesia, and an increase of CD64 at day 1 after anaesthesia. The use of a CPB was followed by a significant reduction of CD32, CD16, CD54 and HLA-ABC antigens expression at the end of surgery. One day after surgery these parameters returned nearly to baseline values with the exception of CD54. A monocyte subpopulation, characterized by low CD14, high CD16 and HLA-DR expression (CD14+CD16+HLA-DR++) was found in both groups at each time point, and the percentage of this cell subset decreased from baseline to 24 h. The plasma concentrations of IL-6 and IL-10 increased considerably during CPB. No dynamic changes of IL-1 level due to surgery or CPB were found. We conclude that anaesthesia as well as the use of CPB induced profound alterations in the number of circulating leucocytes, and in the phenotype of monocyte and cytokine production.