Introduction Cardiopulmonary bypass (CBP) is the most widely used method in cardiac surgery. During the CPB procedure, studies are conducted to maintain myocardial perfusion adequacy, reduce oxidative stress caused by immune reactions, and understand the longevity of the procedure. Recently, microRNAs (miRNAs) have come to the fore to understand the changes in the CPB. In vivo studies have shown that many different miRNAs regulate critical signaling molecules including cytokines, growth factors, transcription factors, proapoptotic, and antiapoptotic proteins. Our study aims to investigate the changes of miR‐34a, miR‐15a, and miR‐320a gene expression in extracorporeal circulation. Methods Fifteen patients who underwent elective open‐heart surgery were included in the study. Serum plasma samples were taken from the patients preoperatively, at the time of CPB, and at 24 h postoperatively. Gene expression of miR‐34a, miR‐15a, and miR‐320a in plasma samples was studied. Differences in gene expression were compared. Results miR‐15a gene expression increased during CPB compared with preoperative levels (p < .001). This increase was decreased after the operation (p < .05). miR‐34a gene expression increased significantly during CPB (p < .01). Similar to the other two gene expressions, miR‐320a gene expression was significantly increased during CPB (p < .01). Conclusions miRNAs may play a key role in the initiation and maintenance of pathophysiological cascades during CPB. Our study showed the gene expression of miR‐34a, miR‐15a, and miR‐320a in the CPB process. Our study will be a pioneer among future studies to investigate the molecular pathophysiology of the CPB process.