Background：Postoperative delirium (POD) is a common complication following cardiac surgery. Mitochondrial injury, demonstrated by excessively activated oxidative stress and regulating cell apoptosis, has been reported to contribute to POD. [Mitophagy](https://www.geenmedical.com/article?id=34752757&type=true), apoptosis, pyroptosis, ferroptosis had been known to play a critical role in degenerative nervous system diseases. However, the serum change of Smac/DIABLO in POD induced by cardiac surgery/anesthesia is still undetermined. We designed the study to determine the expression level of SMAC/DIABLO in POD patients. Methods：A nested case-control study was performed, including 21 POD patients and a matched group of 63 non-POD controls.