Late potentials are an index of gravity following myocardial infarction, but there has been little investigation of the effects of fibrinolysis on their incidence. Eighty-two consecutive patients (68 men, 14 women, group men age = 55 +/- 8 years) admitted presenting with a primary infarction and who had received fibrinolytic treatment within the first four hours. Each patient was screened for late potentials, and underwent a coronary artery angiograph and determination of the left ventricular ejaculation fraction (LCEF) following the infarction. The incidence of late potentials was 38% in the patients with an occluded coronary artery (9/24) vs 12% in patients with a permeable artery (7/58) (p less than 0.02). The change in LVEF was greater if reperfusion was not achieved (47 +/- 13% vs 54 +/- 12%, p less than 0.05). This study suggests that following fibrinolysis, the incidence of late potentials is correlated with both an absence of coronary reperfusion and deterioration of left ventricular function.