This study investigates the impact of preoperative anemia on postoperative outcomes in cardiac surgery, particularly focusing on acute kidney injury (AKI) and hospital stay duration. Analyzing over 4,000 patients, the findings reveal that preoperative anemia significantly increases the risk of AKI and extends hospital stays, with a notable portion of these effects directly attributed to anemia rather than mediated through intraoperative red blood cell transfusions. The research underscores the need for further exploration into anemia’s role in postoperative complications and highlights the importance of addressing preoperative anemia in cardiac surgery patients.