Analysis of actuarial cardiac mortality of 295 patients after a first attack of sustained monomorphic ventricular tachycardia followed up for an average of 5.1 years showed that: the aetiology was the main prognostic factor. Patients with a normal left ventricle had a zero 5 year mortality rate (normal heart) or 3% (arrhythmogenic right ventricular dysplasia) compared with 24% in patients with cardiomyopathy and 40% with myocardial infarction: p less than 0.01. Similar results were obtained if sudden death was considered; in patients with left-ventricular disease, the role of pump function assessed by the ejection fraction was essential: the 5 year mortality was 14.5% (cardiomyopathy) and 30% (myocardial infarction) if this was greater than 0.3 compared to 43% and 51% respectively when less than 0.3 (p less than 0.01); the heart rate of spontaneous ventricular tachycardia and that of tachycardia induced during endocavitary investigation had no influence on the mortality. However, the mortality following a first tachycardia with syncope was higher than that following a well tolerated tachycardia (p less than 0.5 if the ejection fraction less than or equal to 0.3 and p less than 0.001 if greater than 0.3).(ABSTRACT TRUNCATED AT 250 WORDS)