Sixty-nine cases of non-ischemic dilated cardiomyopathy were studied prospectively from February 1983 to February 1989 (52 men: 53 +/- 13.5 years of age). There were 6 cases of sustained ventricular tachycardia. Thirty-seven patients were in Class III or IV of the NYHA Classification. In addition to echocardiography, radionuclide studies, cardiac catheterisation and coronary angiography, they all underwent 24 hour Holter monitoring, signal-averaged electrocardiography and, in 46 cases, endocavitary electrophysiological investigations. Holter monitoring showed ventricular extrasystoles greater than or equal to Grade 3 of Lown's classification in 72% of patients (26% had nonsustained ventricular tachycardia) and these patients had a significantly lower cardiac index. Twenty five per cent of patients had late ventricular potentials (versus 2% in 50 normal subjects; p less than 0.02); this proportion rose to 32% in those patients with greater than or equal to Grade 3 ventricular extrasystoles and to 66% in the patients with spontaneous ventricular tachycardia; the cardiac index was lower in patients with late ventricular potentials (2.3 vs 2.8 l/min/m2; p less than 0.01) and they had a higher incidence of greater than or equal to Grade 3 ventricular extrasystoles (94% vs 65% in patients without late ventricular potentials, p less than 0.05). Programmed ventricular stimulation induced sustained or nonsustained monomorphic ventricular tachycardia and ventricular fibrillation in 15% of cases. The 6 cases of induced sustained ventricular tachycardia were only observed in the 6 patients with spontaneous sustained ventricular tachycardia and they had the same electrocardiographic appearances.(ABSTRACT TRUNCATED AT 250 WORDS)