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Abstract: Background: Bundle branch block (BBB) is a powerful independent predictor of cardiovascular mortality in patients with heart failure (HF) and reduced left ventricular ejection fraction (LVEF). The prognostic implications in HF with preserved systolic function (HF–PSF) are less well understood. Methods: The Candesartan in Heart failure: Assessment of Reduction in Mortality and morbidity (CHARM) programme randomised 7599 patients with symptomatic HF to receive candesartan or placebo. The primary outcome comprised cardiovascular death or HF hospitalisation. The relative risk conveyed by BBB relative to a normal electrocardiogram was examined. Results: The prevalence of BBB was significantly lower in patients with preserved compared with reduced systolic function (CHARM-Preserved 14.4%, Alternative 29.6%, Added 30.5%), p <0.0001. Overall, the adjusted hazard ratio for the primary outcome was 1.48 (95% confidence interval 1.22–1.78), p <0.0001, reflecting increased risk in patients with reduced LVEF (1.72 [1.28–2.31], p =0.0003). The apparently more modest risk among patients with HF–PSF was significant in unadjusted (1.80 [1.37–2.37], p <0.0001) but not adjusted analysis (1.16 [0.88–1.54], p =0.2897). However, no formal statistical difference was observed between the two cohorts, and interpretation is limited by the unknown prevalence of left and right BBB morphologies in each. Comparing BBB presence with absence yielded qualitatively similar results. Conclusion: The simple clinical finding of BBB is a powerful independent predictor of worse clinical outcomes in patients with HF and reduced LVEF. It is less frequent, with a more modest predictive effect, in patients with preserved systolic function. [Copyright &y& Elsevier]