Ten consecutively referred patients with paroxysmal supraventricular tachycardia and ten consecutively referred patients with atrial fibrillation underwent untreated surveillance with telephone/ cardiobeeper monitoring for symptomatic arrhythmias and had four 24-hour ambulatory ECGs recorded at weekly intervals to detect symptomatic and asymptomatic arrhythmias. The Cox proportional hazards model was used to test the hypothesis that the mechanism of tachycardia was the most important predictor of the tachycardia-free period during an untreated observation period.