The prevalence on ECG, Holter monitoring and stress testing and the prognosis of ventricular arrhythmias were assessed in 236 patients with dilated cardiomyopathy (DCM). This diagnosis was confirmed by haemodynamic and coronary angiographic criteria in 218 cases. Analysis of ECG recordings confirmed the high incidence of ventricular extrasystoles (VES) which were observed in 56% of cases, with doublets in 28% and salvoes in 5% of cases. Sustained VT was observed in 5 cases before the diagnosis was established and in 9 cases during follow-up. Ventricular fibrillation was documented in 8 patients without overt cardiac failure. Holter monitoring over 24 hours (N = 76) showed less than 30 VES per hour in 32% of cases, greater than 100 per hour in 36% of cases, predominantly diurnal in 36% of cases, doublets in 40% of cases and salvoes in 15% of cases. A significant correlation was observed (p less than 0.01) between repetitive activity and the frequency of VES. Exercise stress testing (N = 113) showed aggravation of the ventricular arrhythmia in 28% cases. Of the 72 clinical and paraclinical variables that were analysed, the group of patients with VES and doublets on ECG had lower left ventricular ejection fractions and higher pulmonary pressures. Eighty patients died of cardiac causes during follow-up of 38.8 +/- 27 months. Taking into account the haemodynamic differences, the presence of doublets on the ECG was a poor prognostic factor in subjects with ejection fractions greater than 40% (p less than 0.06); this was even more significant when greater than 50% (p less than 0.027).(ABSTRACT TRUNCATED AT 250 WORDS)