The third isotype of beta adrenergic receptors (β3 ARs) has distinctly different effects on cardiomyocytes compared with β1 and β2 ARs. Stimulation of β3 ARs may reduce cardiomyocyte Na+ overload and reduce oxidative stress in heart failure (HF). We examined if treatment with the β3 AR agonist mirabegron increases LVEF in patients with HF.