Patients with symptomatic sustained ventricular tachycardia and those who have been resuscitated after primary ventricular fibrillation have a high risk of sudden cardiac death. Prerequisite to a prognostically favorable outcome is treatment documented to be effective. The value of ambulatory ECG monitoring is based on the assumption that a reduction in spontaneous ventricular arrhythmias implies a reduction in the risk of spontaneous tachycardia recurrence or sudden death. The value of programmed electrical stimulation is based on the assumption that noninducibility of malignant arrhythmias during antiarrhythmic treatment conveys a reduced risk of spontaneous tachycardia recurrence or sudden death. Ambulatory ECG monitoring: With the aid of ambulatory ECG monitoring, the occurrence of spontaneous arrhythmias can be registered uninterruptedly, usually over a period of 24 to 48 hours. The procedure carries no risk, it can be performed on an out-patient basis, it is well-tolerated and cost-effective. Malignant tachyarrhythmias generally cannot be found to recur and, subsequently, cannot be taken into consideration for assessment of treatment effects. Ventricular premature contractions, in particular couplets and salvos in acute myocardial infarction are regarded as warning arrhythmias for ventricular fibrillation and have also been shown to be a risk factor for sudden death in the late post-infarction phase. They are used for assessment of treatment effects. Due to the high degree of spontaneous variability of the arrhythmias, statistical models are necessary to delineate a specific reduction during treatment as drug effect and not a random fluctuation.(ABSTRACT TRUNCATED AT 250 WORDS)