Significant problems exist in the interpretation of serum digoxin concentration data. Failure to distinguish between results that do not require precise clinical correlation (proof of absorption, presence of drug, etc) and those which depend upon clinical correlation for their meaning (‘toxicity’ or ‘effectiveness’) can result in interpretive errors. Problems relating to the source of the serum digoxin concentration can also confound interpretation. Such difficulty may be controllable (obtaining the sample at the proper time, haemolysis, etc) or related to the laboratory technique (cross-reactivity with digoxin metabolites or other medications, technical errors, or lack of precision).Variation within the same patient over time or between patients related to disease (alterations in electrolytes, adrenergic or parasympathomimetic tone, or other medications) may prevent the direct attribution of an observed phenomenon to a particular digoxin concentration. Techniques for determining the effect of digoxin do exist and can be used to gather data for clinical correlations. Ways of improving the interpretation of serum digoxin concentrations also exist and should be used to improve their value in patient management.The serum digoxin concentration seems to have an important future role. However, we need to know how better to interpret and exploit serum digoxin concentration data.