Background: Cardiopulmonary bypass (CPB) induces postoperative immunosuppression, including decreased T‑cells and lower plasma regulated on activation, normal T‑cell expressed and secreted (RANTES) concentrations. MicroRNA‑125a negatively regulates RANTES expression in activated T‑cells. The aims were to investigate microRNA‑125a expression in T‑cells and RANTES production following CPB. Materials and Methods: Twenty‑eight patients undergoing elective cardiac surgery were included in this study. Arterial blood was sampled at six sequential points (before anesthesia induction, before CPB, at 2, 4, 6, and 24 h after beginning CPB) for plasma RANTES concentrations by enzyme‑linked immunosorbent assay. T‑lymphocytes were isolated from whole blood at four points (before anesthesia, before CPB, at 2 and 4 h after beginning CPB) for intracellular microRNA‑125a expression by quantitative real‑time reverse transcription polymerase chain reaction in 14 patients. Perioperative laboratory data and variables were also recorded. Results: The plasma RANTES concentrations decreased significantly at 2–24 h after beginning CPB, with concurrent reduction of postoperative lymphocyte counts, as compared with the preanesthesia level (P < 0.001). Intra‑T‑cell microRNA‑125a expression was activated at 2 and 4 h, however, without significance (P = 0.078 and 0.124, respectively). The plasma RANTES levels at 4 h were not correlated with CPB time (P = 0.671), anesthesia time (P = 0.305), postoperative extubation time (P = 0.508), and Intensive Care Unit (ICU) stay (P = 0.756). Three patients expired with pneumonia‑ or mediastinitis‑related septic shock in the ICU. Conclusion: Plasma RANTES concentrations were depressed till 24 h following CPB, with reduced lymphocytes after cardiac surgery. MicroRNA‑125a expression in T‑lymphocytes was not correlated with perioperative variables and its role in downregulation of RANTES production needs to be determined.