Increased QT dispersion, which has been proposed as a marker of ventricular repolarization inhomogeneity, may predispose to ventricular arrhythmias. Data on QT dispersion in patients with congestive heart failure are scarce. In this study, conventional 12-lead ECGs were recorded in 135 consecutive patients with congestive heart failure secondary to idiopathic dilated cardiomyopathy. Seventy-five patients were excluded from QT interval assessments due to one or more of the following reasons: (1) low amplitude of the T wave (n = 3), (2) atrial fibrillation (n = 26) and (3) bundle branch block (n = 46). QT dispersion was calculated as (1) QT-range: the difference between the maximum and minimum QT intervals on any of the 12 leads and (2) QT-SD: the standard deviation of the QT interval in all the 12 leads. RR intervals were measured in leads II, aVL, V2 and V5. QT-SD (20.85 +/- 5.00 ms) was significantly (r = 0.8997, P < 0.001) related to QT-range (65.65 +/- 15.77 ms), but not to the QT interval. Neither QT-range nor QT-SD was significantly related to age, left ventricular dimensions, left ventricular end diastolic pressure, left ventricular ejection fraction or left ventricular wall thickness. There was no significant difference in QT dispersion between survivors and those who died (n = 8) or were transplanted (n = 9) during 34 +/- 23 month follow-up. No significant difference in QT dispersion was observed between patients with and without ventricular tachycardia (> or = three consecutive beats) detected on 24-h Holter ECGs. RR interval variation was significantly lower in patients who died compared with survivors (standard deviation: 10.37 +/- 3.61 vs 36.02 +/- 35.03 ms, P < 0.001; coefficient of variance: 1.87 +/- 0.7% vs 4.50 +/- 4.9%, P = 0.001). This was also true in patients with bundle branch block. These observations suggest that QT dispersion in idiopathic dilated cardiomyopathy is not significantly related to either QT interval or cardiac size and function and does not predict death. The application of QT dispersion assessment is limited by the commonly encountered atrial fibrillation and bundle branch block in this patient population. However, reduced RR variation on standard 12-lead ECGs has important prognostic implications in these patients.