Hypothesis: Concentrations of bioactive oxidized lipids, such as fragmented oxidized phosphatidylcholines (OxPCs) and oxylipins, will increase significantly following ischemia/reperfusion (I/R) and changes in oxidized lipids concentrations are correlated with markers of tissue injury. Objective: The goal of this thesis was to determine the changes of bioactive oxidized lipids, including OxPCs and oxylipins during I/R. Methods: Kidney I/R was induced in male Sprague–Dawley rats by clamping the left renal pedicle for 45 min followed by reperfusion for either 6h or 24h. For myocardial I/R, blood samples were collected from patients presenting with ST-Elevation Myocardial Infarction STEMI before primary percutaneous coronary intervention (PPCI) (Isch) and at 4 time-points post-PPCI, including 2h (R-2h), 24h (R-24h), 48h (R-48h) and 30 days (R-30d). As controls, blood samples were collected from age-matched patients with non-obstructive coronary artery disease. OxPCs and oxylipins were identified and quantitated using high-performance liquid chromatography-mass spectrometry (HPLC-MS/MS). Result: During renal I/R, total tissue concentrations of fragmented aldehyde-containing OxPCs were significantly elevated at 6h and 24h post-reperfusion compared to sham-operated animals (p<0.05) and their levels were significantly correlated with plasma creatinine levels (r=0.888, p=0.001). In patients with STEMI, total plasma levels of fragmented OxPCs increased significantly during ischemia compared with controls (p<0.05). Their levels remained elevated for 48h after reperfusion and then decreased significantly at 30-day post-MI compared with R-2h and R-24h groups (p<0.05). STEMI patients with an elevated peak of creatine kinase (CK) concentrations had significantly higher ischemic plasma levels of 1-palmitoyl-2-(5'-oxo-valeroyl)-sn-glycero-3-phosphocholine (POVPC) and 1-palmitoyl-2-(9-oxo-nonanoyl)-sn-glycero-3-phosphocholine (PONPC) (p<0.05). Plasma concentrations of total docosahexaenoic acid (DHA) derived oxylipins were significantly elevated in the Isch group compared with controls (p<0.001). Moreover, their levels during ischemia were significantly correlated with the peak CK and troponin T(TnT) levels (p<0.05). Univariate receiver operating characteristic (ROC) curve analysis showed that the higher ratio of epoxides to diols during the ischemia could be considered as a potential marker of having a smaller infarct size based on CK levels (AUC= 0.77, p=0.03). Conclusion: We have shown for the first time that bioactive oxidized lipids are produced during I/R and their levels correlate with markers of tissue injury.