Digoxin has been used to treat congestive heart failure (CHF) for more than two centuries. It's clinical efficacy, however, has been under question in recent years because recent clinical trials showed that digoxin therapy in CHF patients was associated with no beneficial effects in mortality, but only a modest reduction in clinical symptoms and the frequency of heart failure related hospitalisation. Digoxin's effect on mortality seems closely related to its serum concentrations; high serum concentrations (e.g. greater-or-equal, slanted1.2 ng/ml) have been found to increase the risk of all-cause mortality in heart failure patients. Digoxin-associated risk in mortality may be due to an increases in myocardial oxygen consumption and arrhythmogenesis at higher serum concentrations. We hypothesized that the serum concentration of digoxin is a major determinant factor of its efficacy on mortality rates in patients with congestive heart failure. The maintenance of digoxin's serum concentration at the lower end of the reference range, i.e., between 0.5 and 0.8 ng/ml, may reduce mortality rates as well as improve clinical symptoms.