The prevalence of ventricular arrhythmias was evaluated in 35 patients with severe congestive heart failure (CHF) in New York Heart Association functional class III to IV. The etiology of CHF was equally distributed between ischemic and nonischemic cardiomyopathy. The severity of cardiac dysfunction was evidenced by left ventricular ejection fraction of less than 20%, mean cardiac index of 1.75 +/- 0.40 L/min/m2, pulmonary capillary wedge pressure of 28.1 +/- 7.1 mm Hg, and mean exercise capacity of 6.0 +/- 3.6 minutes. During 24-hour ambulatory Holter monitoring, 71% of these patients demonstrated repetitive episodes of ventricular tachycardia (VT), 92% had multifocal ventricular ectopic beats, and 88% had greater than or equal to 10 ventricular ectopy/1000 normal heart beats. Within 1 to 72 weeks of the Holter monitoring 25 patients died. Death could be attributed to VT in only one patient. In all the others, death was secondary to worsening CHF. Thus, although asymptomatic malignant ventricular arrhythmia occurred frequently in our patients, sudden death was rarely observed.