During 1976, 24-hour ambulatory electrocardiographic (ECG) monitoring was available to all physicians at this hospital, and 281 patients were investigated by 322 recordings. Cardiac arrhythmias requiring treatment were detected in 100 patients (36%). Some presented after symptoms such as faintness, giddiness, palpitations, collapse, or fits, but ominous arrhythmias were also found in asymptomatic patients. A demand pacemaker was implanted for episodic sinoatrial or atrioventricular conduction disorder in 30, while 70 patients (25%) required antiarrhythmic drug treatment for ventricular or atrial tachyarrhythmias. Facilities for ambulatory 24-hour ECG monitoring are necessary in any large hospital, and precise diagnosis in most of our patients studied could not have been achieved by any other investigation.