24-hours Holter's ecg monitoring was performed in 79 patients with the hypertrophic cardiomyopathy. Ventricular arrhythmias were recorded in 75.9% of patients, including those revealed in the second recording in 29.5% of patients. Subsequent ecg monitoring was especially useful in detection of complex forms of ventricular arrhythmias (in 27.5%). Average doses of verapamil and Beta-adrenergic blockers were not sufficient in the antiarrhythmic therapy. There was no close correlation between ventricular arrhythmias and a morphologic type of a cardiomyopathy. Patients with episodes of fainting or loss of consciousness require repeated Holter's ecg monitorings.