The QRS duration can be easily obtained from a 12-lead electrocardiogram. Increased QRS duration reflects greater ventricular activation times and often ventricular dyssynchrony. Dyssynchrony causes an impairment of the global cardiac function and adversely affects the prognosis of patients with heart failure (HF). Little is known about the impact of pharmacologic therapies on the QRS duration, particularly for patients with presymptomatic HF with a preserved left ventricular (LV) ejection fraction (i.e., stage B HF with preserved ejection fraction [HFpEF]). The HOMAGE (Heart OMics in AGEing) trial enrolled patients at risk factors for developing HF and assigned them to receive either spironolactone or the usual care for approximately 9 months in a randomized manner. This analysis reports the effect of spironolactone on the QRS duration. A total of 525 patients was included in the analysis. The median (percentile25-75) QRS duration at baseline was 92 (84 to 106) ms. Spironolactone reduced the QRS duration at month 9 by -2.8, 95% confidence interval -4.6 to -1.0 ms, p = 0.003. No significant associations were found between month 9 changes in the QRS duration and corresponding changes in the LV ejection fraction, LV mass, LV end-diastolic volume, blood pressure, N-terminal pro-brain natriuretic peptide, and procollagen type I carboxy-terminal propeptide (all p >0.05). This analysis shows that for patients with stage B HFpEF, therapy with spironolactone for 9 months shortened the QRS duration, an effect that was not associated with reductions in LV mass or volume, supporting the hypothesis that spironolactone has direct beneficial effects to improve myocardial electrical activation in patients with stage B HFpEF.