To examine the relationship between early arrhythmias, infarct size and prognosis, we compared 22 consecutive patients surviving acute myocardial infarction (AMI) and primary ventricular fibrillation (VF) with a control population after AMI uncomplicated by primary VF. Left ventricular ejection fraction (EF) was measured by radionuclide ventriculography before discharge from hospital. Mean EF was significantly reduced below normal following AMI with or without primary VF (normal 0.57 +/- 0.05, mean +/- SD; P less than 0.01). Mean EF was lower among patients who survived primary VF than among those with infarction uncomplicated by primary arrhythmia (0.33 +/- 0.12 v. 0.46 +/- 0.07; P less than 0.01). There were striking differences in EF between those patients with anterior and those with inferior infarction. Mean EF for those surviving primary VF after transmural anterior infarction (0.23 +/- 0.06) was lower than those who had primary VF after transmural inferior infarction (0.43 +/- 0.06; P less than 0.01). Normal left ventricular function was seen in four individuals who developed no further complications. Recurrent primary ventricular arrhythmia was seen only in those individuals subsequently shown to have reduced EF. Low EF (less than 0.35) was seen in 12 patients with primary VF in the context of anterior infarction, five developed breakthrough ventricular arrhythmias despite therapy and in a limited follow-up period, three have died.