This study investigates the link between plasma-free haemoglobin (PFH) and acute kidney injury (AKI) in children undergoing cardiopulmonary bypass (CPB) during cardiac surgery. Among 179 participants, 41% developed AKI. Peak PFH concentrations were significantly higher in patients with AKI, but multivariable analysis showed CPB duration and patient age, rather than PFH, as primary risk factors. This emphasizes CPB time management in mitigating AKI risk.