Signal-averaged electrocardiograms (ECGs) were registered before hospital discharge in 120 consecutive patients (pts) with first acute myocardial infarction (MI). 26 pts (22%) had abnormal signal-averaged ECGs - late potentials (LP). Pts with LP had a more serious clinical course than pts without LP: sustained ventricular tachycardia (VT) (27% vs. 2%; p less than 0.01), left ventricular aneurysm (35% vs. 11%; p much less than 0.05) and greater infarct size (62 +/- 35 vs. 43 +/- 22 CK-MB gEq; p less than 0.01). Complex ventricular ectopic activity was more often in pts with LP than in those without LP (54% vs. 28%; p less than 0.05). During an up to 18-month follow-up (mean 13) 6 pts presented sustained VT and 2 died suddenly. 5 pts with sustained VT and 2 pts who died suddenly had LP. The sensitivity of the signal-averaged ECG as a predictor of arrhythmic events was 86%, with a specificity of 82%. Signal-averaged ECGs provide prognostic information in identifying arrhythmic events after MI.