Primary ventricular fibrillation (PVF) occurs in approximately 3--10% of uncomplicated acute myocardial infarction (AMI) patients. The major reason for this variability in incidence appears to be population diferences in the time from symptom onset to admission. Other risk factors have not been consistently shown to influence the risk of PVF. Warning arrhythmias do not warn of impending PVF in from 20--80% of AMI patients; thus, antiarrhythmic therapy reserved for those with warning arrhythmias may fail to prevent PVF in a significant number of patients. Although a review of 13 controlled trials of lidocaine prophylaxis shows only two suggesting a protective effect from this drug, only one study was free of major defects in trial design. This trial showed a striking decrease in PVF incidence when lidocaine was employed. Lidocaine should probably be administered to all uncomplicated AMI patients during the first 48 hours after infarction. Its utility in preventing ventricular fibrillation in complicated AMI patients and in the very early AMI period is unclear.