Death in chronic heart failure (CHF) can be from progression of disease or sudden and unexpected. We have attempted to identify factors that predict sudden death in CHF. We followed up 44 patients with CHF for 12-50 (mean 36) months. 4 patients died of non-cardiovascular causes and were excluded from analysis. There were 7 sudden deaths (symptoms for less than 1 h in a previously stable patient) and 12 from progressive CHF. Patients who died of progressive CHF had lower left-ventricular ejection fractions and higher concentrations of atrial natriuretic factor than the 21 survivors, but there were no differences in these variables between survivors and those who died suddenly. However, the sudden death group had significantly (p<0·05) greater inter-lead variability in the QT interval on the electrocardiogram (QT dispersion; 98·6 [95% Cl 79·1-118] ms^1/2) than survivors (53·1 [41·9-64·3] ms^1/2) or the group who died from progressive CHF (66·7 [51·8-81·6] ms^1/2). QT dispersion is a marker of myocardial electrical instability. The association of increased QT dispersion with sudden death suggests that patients at high risk of such death could be identified by means of this simple, reproducible test. This group might benefit from more intensive treatment.