A multifactorial analysis was performed to study the factors that contributed to the occurrence of late potentials on the signal-averaged electrocardiogram in 106 consecutive patients with a first myocardial infarction. Ninety-three (88%) patients received intravenous thrombolytic therapy within 6 hours of symptom onset. Thirty-two (30%) patients had a late potential on the signal-averaged electrocardiogram on day 6, including 17 of 31 (55%) in whom the infarct-related artery was occluded and 15 of 75 (20%) in whom it was patent (P = 0.0004). Twenty-three variables were analyzed by a multifactorial stepwise regression analysis. Predictors of a late potential were (1) an occluded infarct-related coronary artery (t = -3.653, P = 0.0004) and (2) the extent of myocardial necrosis as indicated by the peak serum lactate dehydrogenase level (t = 3.094, P = 0.0025). The lower incidence of late potentials when the infarct-related coronary artery was patent was independent of left ventricular ejection fraction and peak enzyme levels after infarction.