The prognostic value of ventricular arrhythmias on the natural history of idiopathic dilated cardiomyopathy remains controversial. Our findings in 105 patients suggest that those with reduced left ventricular ejection fraction (less than 40%) in whom frequent episodes of ventricular tachycardia or ventricular pairs (greater than 10 to 20/24 hours) were detected by 24 hour Holter monitoring are at risk of sudden death.