Aims: Left ventricular mechanical dyssynchrony has been described in heart failure with preserved ejection fraction (HFpEF), but its prognostic significance is not known. Methods and results: Of 3445 patients with HFpEF enrolled in the Treatment of Preserved Cardiac Function Heart Failure with an Aldosterone Antagonist (TOPCAT) trial, dyssynchrony analysis was performed on 424 patients (12%) by multiple speckle tracking echocardiography strain-based criteria. The primary dyssynchrony analysis was the standard deviation of the time to peak longitudinal strain (SD T2P LS). Cox proportional hazards models assessed the association of dyssynchrony with the composite outcome of cardiovascular death or heart failure hospitalization. Mean age was 70 ± 10 years, LVEF was 60 ± 8%, and QRS duration was 101 ± 27 ms. Worse dyssynchrony, reflected in SD T2P LS, was associated with wider QRS, prior myocardial infarction, larger LV volume and mass, and worse systolic (lower LVEF and global longitudinal strain) and diastolic (lower e' and higher E/e') function. During a median follow-up of 2.6 (interquartile range 1.5–3.8) years, 107 patients experienced the composite outcome. Worse dyssynchrony was associated with the composite outcome in unadjusted analysis [hazard ratio (HR) 1.04, 95% confidence interval (CI) 1.01–1.07; P = 0.021, per 10 ms increase], but not after adjusting for clinical characteristics, or after further adjustment for LVEF, AF, NYHA class, stroke, heart rate, creatinine, haematocrit, and QRS duration (HR 1.03, 95% CI 0.99–1.06; P = 0.16, per 10 ms increase). Conclusion: Worse LV mechanical dyssynchrony, assessed by speckle tracking echocardiography, is not an independent predictor of adverse outcomes in HFpEF, suggesting that mechanical dyssynchrony is unlikely to be an important mechanism underlying this syndrome. These findings warrant validation in an independent study specifically designed to assess the prognostic utility of mechanical dyssynchrony in HFpEF. Trial registration: NCT00094302.