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Background--Recently, we reported that urinary 8-hydroxy-2'-deoxyguanosine (U-8-OHdG), an oxidative stress marker, reflected inflammatory activity in cardiac sarcoidosis (CS). Here, we investigated whether U-8-OHdG levels were associated with ventricular tachycardia (VT) in patients with CS. Methods and Results--This prospective cohort study enrolled 62 consecutive patients with CS, of whom 36 were diagnosed as having active CS based on abnormal 18F-flurodeoxyglucose accumulation in the heart on positron-emission tomography/computed tomography. The 36 patients with active CS were subdivided as having CS with sustained VT (CS-VT group; n=18) or CS without sustained VT (CS-nVT group; n=18). Twenty-seven patients diagnosed with idiopathic dilated cardiomyopathy served as heart failure controls. U-8-OHdG, brain natriuretic peptide, cardiac function indices, and immunohistological data from subendomyocardial biopsy samples were compared across groups. Immunohistochemical examination of ventricle biopsy samples revealed that the anti-8-OHdG antibody-positive area of cardiac tissue was significantly greater in CS-VT than in CS-nVT or dilated cardiomyopathy and significantly correlated with U-8-OHdG levels (n=58; R=0.61; P<0.00001), which were significantly higher in CS-VT than in CS-nVT (24.6±7.1 versus 15.2±3.8 ng/mg⋅Cr; P<0.0001). Other baseline characteristics did not differ between the groups. Multivariate analysis indicated that U-8-OHdG was an independent determinant factor for VT. Receiver operating characteristic curve analysis to identify patients with VT resulted in a U-8-OHdG cutoff value of 17.5 ng/mg ⋅Cr (sensitivity, 89%; specificity, 83%; area under the curve, 0.90). Conclusions--U-8-OHdG levels are associated with VT in patients with active CS diagnosed by 18F-flurodeoxyglucose positron-emission tomography, providing additive and relevant information about the arrhythmia substrate. [ABSTRACT FROM AUTHOR]