Clinical studies have solidified the utility of digoxin in patients with left ventricular dysfunction and normal sinus rhythm. No definitive data have been published to clarify the range of serum digoxin concentrations associated with clinical benefit. The traditional therapeutic range of 0.8-2.0 ng/ml was developed originally to classify digoxin toxicity, not efficacy. In addition, this reference range was used before publication of the Digitalis Investigators Group trial. Clinical and neurohormonal studies have attempted to characterize serum concentrations that are associated with clinical efficacy.