Practice guidelines recommend that serum potassium levels be maintained between 4.0 and 5.0 mEq/L in patients with acute myocardial infarction (MI). However, this guidance largely derives from small studies that predate modern management of patients with acute MI. In a retrospective cohort study that included 38 689 patients with confirmed acute MI (2000-2008), Goyal and colleagues assessed the relationship between serum potassium levels and hospital mortality. The authors report a U-shaped relationship between mean postadmission serum potassium level and mortality, with the lowest mortality observed among patients whose mean serum potassium levels were between 3.5 and less than 4.5 mEq/L. In an editorial, Scirica and Morrow discuss potassium repletion in patients with acute MI.