The QT variability index (QTVI) indicates temporal dispersion in myocardial repolarization, and a high QTVI is associated with a propensity for sudden death from malignant ventricular arrhythmias in subjects at high risk. In this study, the authors assessed the effects of free breathing, controlled breathing, and sympathetic stress (tilt) on the QTVI in patients with chronic heart failure (CHF) and healthy control subjects. The authors also examined the influence of age on the same variables. To obtain normative data, they calculated 95% confidence intervals for healthy subjects grouped according to age. Under all experimental conditions, the QTVI was larger in the CHF group overall and in the age subsets than in controls. In patients and controls, the QTVI increased significantly during tilt, although no differences were found between the QTVI measured during free and controlled breathing. In healthy controls, the following variables correlated significantly with the QTVI: age and baseline heart rate (P < 0.001). In patients with CHF, aging had no influence on the QTVI. Conclusion: Age, sympathetic stress, and CHF all tend to increase the QTVI and could potentially induce sudden death. Further studies should assess the usefulness of the QTVI as a marker predicting sudden cardiac death under the various conditions of risk.