Cardiac surgery with Cardiopulmonary Bypass (CPB) induces postoperative immunological responses in patients. We aimed to determine changes in various immunological parameters in patients during and after cardiac surgery supported by CPB and to search for possible reliable predictors of postoperative bacterial infections. CD4, CD8, CD3, CD19, CD45, CD16+56+ and CD3+HLA-DR+ lymphocytes and CD14 monocytes were counted by flow cytometry in blood samples taken from 36 patients (21 women and 15 men divided into 3 age groups), three of whom were diagnosed with postoperative infections (fourth group). An increased percentage of B lymphocytes and a reduced percentage of cells with CD4 receptors (T-helper cells) were observed on the first postoperative day. These changes during this period correlated with the onset of bacterial infections.