BACKGROUND In the early 1980s, studies performed in highly selected referral patients with hypertrophic cardiomyopathy reported a strong association between the presence of brief episodes of ventricular tachycardia (VT) on ambulatory ECG monitoring and sudden death. These observations led to antiarrhythmic treatment in many patients with hypertrophic cardiomyopathy and brief episodes of VT. In recent years, however, a growing awareness of the potential arrhythmogenic effects of antiarrhythmic medications has raised doubts regarding such a therapeutic approach, particularly in less selected and lower-risk patient populations. RESULTS In the present study, we examined the prognostic significance of nonsustained VT in a population of 151 patients with hypertrophic cardiomyopathy who were asymptomatic or had only mild symptoms at the time of their initial ambulatory ECG recording. Of the 151 study patients, 42 had episodes of VT and 109 did not. The runs of VT ranged from 3 to 19 beats, with 35 patients (83%) having < 10 beats. The number of runs of VT ranged from 1 to 12 in 24 hours, with 36 patients (86%) having < or = 5 episodes of VT. Thus, in most patients, the episodes of VT were brief and infrequent. Follow-up averaged 4.8 years. Of the 151 study patients, 6 died suddenly, 3 in the group with VT and 3 in the group without VT. Two other patients, both in the group without VT, died of congestive heart failure. The total cardiac mortality rate was 1.4% per year in the patients with VT (95% CI, 0.4% to 3.5%) and 0.9% in those without VT (95% CI, 0.4% to 2.0%; P = .43). The relative risk of cardiac death for patients with VT was 1.4 compared with patients without VT (95% CI, 0.6 to 6.1). The sudden death rate was 1.4% per year in the patients with VT (95% CI, 0.4% to 3.5%) and 0.6% in those without VT (95% CI, 0.2% to 1.5%; P = .24). The relative risk of sudden death for patients with VT compared with those without VT was 2.4 (95% CI, 0.5 to 11.9). Of the 151 patients included in the study, 88 (58%) remained asymptomatic and were not treated with cardioactive medications during follow-up. Of these 88 patients, 20 were in the group with VT and 68 in the group without VT. None of these patients died. CONCLUSIONS Our results show that cardiac mortality is low in patients with hypertrophic cardiomyopathy who are asymptomatic or only mildly symptomatic and have brief and infrequent episodes of VT on ambulatory ECG monitoring. Our findings also suggest that brief and infrequent episodes of VT should not be considered, per se, an indication for antiarrhythmic treatment in such patients.