The prevalence and prognostic significance of ventricular arrhythmias identified on 24 hour ambulatory electrocardiographic monitoring was prospectively assessed in 99 patients with hypertrophic cardiomyopathy. In the absence of antiarrhythmic therapy, high grade ventricular arrhythmias (grade 3 and above) were common; that is they were identified in 66 percent of the patients, including 19 percent with episodes of asymptomatic ventricular tachycardia. Clinical outcome was assessed 3 years after the initial 24 hour ambulatory electrocardiogram. Of the 84 patients who did not undergo ventricular septal myotomy-myectomy, 6 died suddenly or experienced cardiac arrest, 1 died of progressive congestive heart failure and the other 77 have survived without a cardiac catastrophe. The prevalence rate of sudden death or cardiac arrest during the follow-up period was the same (3 percent) in patients with high grade arrhythmias other than ventricular tachycardia (1 of 37) as in those with no or low grade arrhythmias (1 of 29). However, the occurrence of a sudden cardiac catastrophe was significantly more common in patients with asymptomatic ventricular tachycardia of brief duration on 24 hour electrocardiography (4 [24 percent] of 17) than in patients without ventricular tachycardia (2 [3 percent] of 66) (p less than 0.02). In summary, (1) high grade ventricular arrhythmias are commonly found on continuous 24 hour electrocardiography monitoring in patients with hypertrophic cardiomyopathy; and (2) although sudden death is relatively uncommon in patients with high grade ventricular arrhythmias other than ventricular tachycardia (annual mortality rate 1 percent), the finding of ventricular tachycardia on 24 hour electrocardiography identifies a subgroup of patients at high risk for sudden death (annual mortality rate 8.6 percent). Although no conclusions can be drawn regarding the impact of therapy, our findings suggest that 24 hour electrocardiographic monitoring should be performed in patients with hypertrophic cardiomyopathy and that it may be reasonable to initiate antiarrhythmic therapy if ventricular tachycardia is identified.