DNA in the nucleus is one of the major targets of reactive oxygen species (ROS), and oxidative DNA damage has been implicated in the pathogenesis of chronic heart failure. 8-Hydroxy-2'-deoxyguanosine (8-OHdG) is produced from deoxyguanosine in DNA by ROS. The purpose of this present study was to examine the clinical significance of serum 8-OHdG levels in patients with heart failure.We measured serum 8-OHdG levels in 230 patients with chronic heart failure and 42 control subjects without heart failure by sandwich enzyme-linked immunosorbent assay. Patients were prospectively followed during a median follow-up period of 472 days with the end points of cardiac death or progressive heart failure requiring re-hospitalization.Serum 8-OHdG concentrations were higher in patients with heart failure than in control subjects (P 0·40 ng mL(-1)) were observed in 21·2%, 43·1%, 42·6% and 69·4% through NYHA I to IV (P < 0·001). A total of 66 cardiac events occurred during a follow-up period, and Kaplan-Meier survival curves demonstrated that cardiac event rate was markedly higher in patients with high 8-OHdG levels than in those with normal 8-OHdG levels (62·4% vs. 29·6%, P = 0·0007).Serum 8-OHdG levels provide important prognostic information for the risk stratification of patients with heart failure.