Seventy-two patients with hypertrophic cardiomyopathy were evaluated by 24 hours of electrocardiographic ambulatory monitoring. The results were related to echocardiographic extent of hypertrophy, left ventricular outflow obstruction at rest, and transmitral flow parameters. Also, the symptoms and functional classification of NYHA were related to ventricular arrhythmias. Ventricular arrhythmias were detected in 68% of our patients: high degree of ventricular ectopia in 26 (36.1%), and ventricular tachycardia in 14 (19.4%). Most important supraventricular arrhythmias were: atrial fibrillation in 7 patients, atrial flutter in one, and supraventricular tachycardia in five. Asymmetric hypertrophy type I of Maron classification showed the lower number of ventricular arrhythmias. High degree of ventricular arrhythmias were more frequent in patients with: apical hypertrophy, worse functional classification of NYHA (I: 20%; II: 50%; III: 80%), and in patients with impaired distensibility detected by Doppler (80.7% vs 56.7%; p less than 0.05). There was no relationship between left ventricular outflow obstruction at rest and ventricular arrhythmias.