We assessed incidence and predictors of hyperkalemia ( potassium 5.5 mmol/L ) and hypokalemia ( potassium < 3.5 mmol/L ) and the relationship to outcomes in 1663 patients with class III or IV heart failure and left ventricular ejection fraction < 35 % randomized to treatment with spironolactone 25 mg or placebo in the Randomized Aldactone Evaluation Study ( RALES ) trial .