Among ambulatory patients with rhythm disturbances, selection of those who should be treated is essential. Type and frequency of arrhythmia cannot be considered in isolation from the underlying heart disease. Aims of treatment are improvement of symptoms caused by the arrhythmia and prevention of sudden death. Except for atrial fibrillation in the WPW-syndrome, supraventricular arrhythmias are most frequently treated because of their hemodynamic consequences. Frequent complex premature ventricular beats may be an indication for antiarrhythmic therapy in certain patients with coronary heart disease, since this rhythm disturbance appears to increase the risk of sudden death in this setting. Similarly, premature ventricular beats may have prognostic significance for subsets of patients with hypertrophic cardiomyopathy, mitral valve prolapse and long Q-T syndrome. In general, however, only a small percentage of all ambulatory patients with arrhythmias needs to be treated.