Time course and prognostic significance of serial signal-averaged electrocardiograms after a first acute myocardial infarction. The prognostic significance of serial signal-averaged electrocardiograms recorded during the first 3 days (period 1), in the second week (period 2) after a first acute myocardial infarction (AMI) and 6 months later (period 3) was prospectively assessed in 190 patients. No patients were treated with thrombolytic therapy. Patients with conduction disturbances were excluded. Mean age of the 190 patients was 57 years (range 34 to 74) and mean left ventricular ejection fraction was 40 + 6% (range 12 to 70). Eighty-four patients had an anterior wall AMI and the remaining 106 patients an inferior wall AMI. After a mean follow-up of 24 months, 16 patients developed sustained symptomatic monomorphic ventricular tachycardia, 7 patients were resuscitated from an episode of ventricular fibrillation, and 10 patients died suddenly. Multivariate regression analysis using continuous variables showed that the strongest predictor of sustained ventricular tachycardia and ventricular fibrillation was the left ventricular ejection fraction (p less than 0.0001) followed by the duration of QRS complex on the signal-averaged electrocardiogram recorded during the first 3 days of AMI (p less than 0.0005). Sudden death was only predicted by left ventricular ejection fraction (p less than 0.02). 