BACKGROUND: Inflammation and oxidative stress are thought to play an important role in the pathophysiology of heart failure with preserved ejection fraction (HFpEF) through the development of endothelial dysfunction. Myeloperoxidase (MPO) functions as a link between oxidative stress and inflammation and is an interesting therapeutic target. The objective of this observational cohort study was to compare MPO levels between HFpEF and old controls, to define clinical characteristics associated with high levels of MPO and to assess the relation between MPO levels and vascular function. METHODS: Patients with HFpEF (N = 55) and controls 60 years (N = 18) were prospectively included. All subjects underwent complete echocardiography and blood sampling. MPO levels were dosed by ELISA assay. Effective arterial elastance (Ea) and peripheral arterial tonometry (EndoPAT reactive hyperemia index RHI and augmentation index AIx) were used to assess vascular function. Characteristics between groups defined by the median of MPO were compared using independent samples -test or chi square test. RESULTS: Patients with HFpEF (80 8.7 years, 65% female) had higher levels of MPO compared to controls (75 5.0 years, 72% female) (34.7 ng/mL [22.7; 44.0] vs 22.6 [18.2; 32.0], = 0.026). MPO levels were correlated with markers of inflammation; C-reactive protein (Pearson's R = 0.46, = 0.001) and neutrophile to lymphocyte ratio (R = 0.36, = 0.031) and with signs of left ventricular (LV) remodelling and elevated filling pressures, namely NT-proBNP levels (R = 0.32, = 0.019), decreased LV ejection fraction (LVEF, R = -0.36, = 0.008) and E/e' ratio (R = 0.35, = 0.011). HFpEF patients with levels of MPO above the median were more often men (48% vs 21%, = 0.037) and suffered more often from diabetes (48% vs 18%, = 0.017). Intriguingly, they had lower indices of vascular stiffness (augmentation index 11.1 [0.1; 30.7] vs 19.9 [10.5; 33.4], = 0.018 and arterial elastance Ea 2.06 0.676 vs 2.43 0.721, = 0.065) and there was no difference in endothelial function (1.82 [1.34; 2.30] vs 1.66 [1.32; 1.95], = 0.55). CONCLUSIONS: HFpEF patients have higher levels of MPO than controls, reflecting leukocyte activation and oxidative stress. Among patients, high levels of MPO are associated with male sex, diabetic status, subtle left ventricular dysfunction and pronounced diastolic dysfunction. The association between oxidative stress and vascular stiffness, on the other hand could not be demonstrated. CLINICAL TRIAL REGISTRATION: Clinical trial NCT03197350.