OBJECTIVE--To evaluate changes in QT dispersion and components of the QT interval in patients admitted with unstable angina and acute myocardial infarction and to study the dynamics of these changes in patients with infarction. METHODS--Prospective study recording electrocardiograms at 50 mm/s in patients admitted with typical cardiac chest pain. Subsequent confirmation of acute myocardial infarction according to standard criteria. Single blind analysis for QT dispersion and QT components using a digitiser and simple computer program. Results are expressed as native QT dispersion, QTc dispersion, and the QT dispersion ratio defined as QT dispersion divided by cycle length and expressed as a percentage. RESULTS--QT dispersion, QTc dispersion, and QT dispersion ratio were all higher in patients with acute myocardial infarction than in those with unstable angina (mean (SD) 66 (18) ms, 75 (26) ms1/2, and 8.1 (2.4)% compared with 38 (13) ms, 39 (13) ms1/2, and 4.5 (1.7) % respectively). Dynamic changes in QTc dispersion were seen after acute infarction with significant differences in the QT components occurring between the different patient groups. Levels of QT dispersion (87 (15) ms), QTc dispersion (105 (17) ms1/2), and QT dispersion ratio (11.7 (0.8)%) in the four patients with ventricular fibrillation were significantly higher. Use of QT dispersion ratio gave a narrower confidence interval. CONCLUSION--QT dispersion is increased after myocardial infarction and levels are higher in patients with ventricular fibrillation. The changes in QT dispersion are dynamic and may reflect the changing pattern of underlying ventricular recovery of ventricular excitability, which is profoundly disturbed in the earliest phase of acute infarction. Expressing QT dispersion as a percentage of cycle length (QT dispersion ratio) rather than using standard rate correction may be superior in identifying patients who develop ventricular fibrillation.