OBJECTIVE--To review the importance of heart rate variability analysis in left ventricular dysfunction and heart failure and to assess the effects of drug treatment. In patients with left ventricular dysfunction or heart failure, a low heart rate variability is a strong predictor of a low probability of survival. Because drug treatment in these patients has rapidly changed over the past two decades, the effect of these drugs on heart rate variability needs special attention. DESIGN--A study of published reports to give an overview of heart rate variability in patients with left ventricular dysfunction or heart failure and how it is affected by drug treatment. RESULTS--Analysis of heart rate variability provides an easily obtained early marker for progression of disease. It seems to be more closely related to the degree of neurohumoral activation than to haemodynamic variables. Cardiovascular drugs may either stimulate or inhibit the degree of neurohumoral activation, and the effects of pharmacological intervention can be closely monitored with this method. CONCLUSIONS--The analysis of heart rate variability, including spectral analysis, is a novel non-invasive way to obtain potentially useful clinical information in patients with reduced left ventricular function. The effects of drug treatment on heart rate variability are in general consistent with their long-term effects in left ventricular dysfunction and heart failure.