Abstract--Evidence is available that in heart failure, cardiac resynchronization therapy by biventricular pacing improves myocardial function and exercise capacity. Whether this is accompanied by a sustained inhibition of heart failure-dependent sympathoexcitation is uncertain. In 11 heart failure patients (mean±SEM age, 68.4±1.5 years) in New York Heart Association (NYHA) class III and IV under medical treatment with an intraventricular conduction delay (QRS duration 130 ms), with a markedly depressed left ventricular ejection fraction, and undergoing implantation of a biventricular pacemaker, we measured beat-to-beat blood pressure and muscle sympathetic nerve traffic. Measurements, which also included echocardiographic and clinical variables, were performed before and 10 weeks after successful resynchronization therapy. Ten age- and NYHA class-matched heart failure patients who were under medical treatment for the same time period served as controls. Long-term resynchronization therapy improved cardiac function and caused a significant increase in systolic blood pressure coupled with an improvement in maximal oxygen consumption and exercise capacity. These effects were coupled with a significant and marked reduction in sympathetic nerve traffic when expressed both as burst frequency over time (44.1±3.6 vs 30.7±3.0 bs/min, -30.5%, P<0.02) and as burst frequency corrected for heart rate (68.3±5.9 vs 47.3±4.3 bs/100 beats, -32.1%, P<0.02). No significant change in the aforementioned parameters was seen in the control group. These data provide the first direct evidence that in severe heart failure, resynchronization therapy exerts a marked and sustained sympathoinhibition. Because in heart failure sympathetic overactivity adversely affects prognosis, this may have important clinical implications.