To elucidate the incidence and clinical significance of ventricular late potentials (LP) and reduced heart rate variability (HRV) in primary and secondary heart muscle disease, 157 patients with dilated cardiomyopathy (DCM, n = 19), chronic myocarditis (MC, n = 50), hypertrophic cardiomyopathy (HCM, n = 27) and systemic hypertension (HT, n = 61) were studied. LP measured by the signal averaging technique were found in 24% of the total study group--47% of the patients with DCM, 28% with MC, 29% with HCM and 10% with HT. Complex ventricular arrhythmias were detected during Holter monitoring in 56% of patients with DCM, in 41% with MC, in 21% with HT and in 16% with HCM. An electrophysiological study was performed in a total of 75 patients. Non-sustained or sustained ventricular tachycardia (VT) or ventricular fibrillation (VF) were inducible during programmed ventricular stimulation in 32% of patients with MC, in 30% with HT, in 20% with DCM and in 17% with HCM. The total duration of the signal-averaged, filtered QRS complex was the only independent predictive factor for severe arrhythmic events and sudden cardiac death. HRV measured in 39 patients were most reduced in patients with DCM (RR interval standard deviation (HRV-SD) 39 +/- 23 ms), followed by 44 +/- 16 ms in patients with HCM, 45 +/- 28 ms in patients with HCM and 67 +/- 51 ms in patients with HT. A significant reduction in the HRV-SD below 30 ms was recorded in 24% of patients measured.(ABSTRACT TRUNCATED AT 250 WORDS)