We have devised a simple method for identifying predisposition to spontaneous sustained ventricular fibrillation (VF) and tachycardia (VT). A standardized protocol of programmed stimulation was applied to 111 control subjects without ventricular disease and with no history of VF or VT (Group I) and to 27 patients with previous myocardial infarction and documented spontaneous (in the absence of evidence of further acute myocardial ischaemia) VF or VT (Group II). The stimulation protocol consisted of single and paired ventricular extra stimuli introduced during ventricular drive at the right ventricular apex and outflow tract, at twice diastolic threshold current intensity and at 20 mA. None of the Group I subjects exhibited VF or sustained (more than 10 s) VT. In contrast sustained arrhythmias were induced in 24 (89%) of Group II patients. We conclude: In our study population, initiation of a sustained ventricular tachyarrhythmia at programmed stimulation was both a sensitive (89%) and specific (100%) indicator for predisposition to spontaneous VF and VT.