The prognosis of monomorphous, sustained, non-iatrogenic ventricular tachycardia was studied in 86 patients followed up for a mean period of 42.8 months. The patients were divided into three groups as follows: group I, 46 patients with myocardial infarction beyond the acute phase; group II, 30 patients with non-ischaemic heart disease; group III, 10 patients without heart disease detectable at angiography and/or echotomography. Fifty patients died during the follow-up period. Forty (80 p. cent) of these deaths (26 in group I, 14 in group II) were due to cardiac causes: heart failure in 24 cases, sudden death in 15 cases, recurrent myocardial infarction in 1 case. The percentage of cardiac death was higher in patients with left ventricular dysfunction (66 p. 100 versus 15.2 p. 100) and when the ejection fraction was below 0.40 (52.9 p. 100 versus 11.8 p. 100). The proportion of sudden death was 66.7 p. 100 when Lown's grade IV ectopic rhythm persisted under treatment, and only 7.7 p. 100 when this was not the case (p less than 0.001). These results confirm that: ventricular tachycardia has a high mortality rate in patients with heart disease; ventricular tachycardia on a "healthy" heart is of good prognosis; left ventricular dysfunction and persistence of repetitive forms under treatment have a high predictive value for cardiac death.