OBJECTIVE: To show whether increased QT dispersion on admission predicts ventricular fibrillation after acute myocardial infarction, and to determine the nature of time related changes in QT dispersion. DESIGN: Prospective cohort study. SETTING: Coronary care units of three teaching hospitals in Newcastle-upon-Tyne over an eight month period. PATIENTS: All had acute myocardial infarction according to World Health Organization criteria. INTERVENTIONS: For all patients, QT dispersion (QTd) and Bazett rate corrected QTc dispersion (QTcd) were measured from a high quality 12 lead ECG recorded on admission at a paper speed of 50 mm/s. In a subset, serial ECGs were recorded regularly to show time related changes in QTcd following acute myocardial infarction. MAIN OUTCOME MEASURES: Occurrence of ventricular fibrillation within the first 24 hours after myocardial infarction. RESULTS: Data collected from 201 patients, 12 of whom (6%) developed ventricular fibrillation within 24 hours. Neither QTd nor QTcd differed between those developing ventricular fibrillation and those who did not: QTd mean (SD), 74 (24) ms (95% confidence interval (CI) 59 to 89) v 66 (24) ms (95% CI 62 to 70), respectively; QTcd, 86 (26) ms(0.5) (95% CI 70 to 102) v 77 (29) ms(0.5) (95% CI 72 to 82), respectively. Significant QTcd changes occurred early after myocardial infarction. CONCLUSIONS: Admission QTd and QTcd do not predict ventricular fibrillation after acute myocardial infarction. There are significant changes in QTcd with time, which may account for this measured lack of correlation.