QT dispersion (defined as maximal QT interval minus minimal QT interval) as assessed on the surface electrocardiogram has been demonstrated to reflect regional inhomogeneity of ventricular repolarization. However, the variability of repeated QT dispersion measurements has not been validated in a prospective study. Thus, the present study is based on the analysis of standard 12-lead surface electrocardiographic (ECG) tracings obtained in 127 persons including 50 subjects without structural heart disease and 77 patients presenting with acute myocardial infarction. RR and QT intervals were measured by means of a digitizer tablet and QT/QTc dispersion was subsequently calculated automatically by PC-based analysis software. Measurements were obtained on 2 separate occasions by the same observer to assess the intraobserver variability. In addition, all tracings were evaluated by a second investigator to determine the interobserver variability. QT dispersion in persons without heart disease averaged 30 +/- 10 ms compared with 56 +/- 24 ms in patients with acute myocardial infarction (p < 0.0001). Patients with infarction who developed ventricular fibrillation within the first 24 hours after admission (11 of 77) had an even larger QT dispersion of 88 +/- 30 ms (p < 0.0001). Repeated measurements of QT dispersion in all 127 subjects revealed a correlation coefficient of 0.91 for both intra- and interobserver variability. Similar results were obtained for repeated determination of QTc dispersion (r = 0.93 and r = 0.90, respectively). When only patients with infarction were considered, correlation coefficients between 0.84 and 0.88 were obtained.(ABSTRACT TRUNCATED AT 250 WORDS)