The aim of the study was to provide a detailed characterization of patients with hypertrophic cardiomyopathy who are at increased risk of severe cardiac rhythm disorders and, thus, also sudden death. The group, made up of 64 patients, was subjected to 24-hour ECG Holter monitoring. The patients were examined by echocardiography and myocardial hypertrophy distribution was studied in detail. A significantly higher incidence of severe ventricular arrhythmias was found in patients with hypertrophy involving large areas of the myocardium and with increased myocardial wall thickness. Patients with a positive history of syncopes were found to have a high incidence of severe supraventricular and ventricular cardiac rhythm disorders. No relationship was established between the incidence of arrhythmias and the presence or extent of obstruction. Patients with extensive hypertrophy (both in terms of the area involved and myocardial thickness) as well as those with a history of syncopes represent a risk group as regards the development of potentially lethal arrhythmias.