In 64 patients with hypertrophic cardiomyopathy 24-hour Holter monitoring of the ECG was made in order to detect the incidence of individual disorders of the cardiac rhythm. The examination revealed a large number of potentially malignant ventricular arrhythmias (47% of the patients). Most frequently polytopic ventricular extrasystoles were involved (31.3%), attacks of ventricular tachycardia (20.3%), less frequently bigeminy was recorded (10.9%) and early extrasystoles type R on T (3.1%). As to other disorders of the cardiac rhythm, paroxysmal supraventricular tachycardia was frequent (34.4%), atrial fibrillation or flutter (9.4%) and more numerous supraventricular extrasystoles (7.8%). Sinoatrial (4.7%) and atrioventricular blocks (7.8%) were relatively rare. Examination of patients with hypertrophic cardiomyopathy is an essential prerequisite for the initiation of early and effective treatment which can exert a favourable effect on the subsequent fate of the patients.