Paroxysmal arrhythmias are a group of disorders in which sudden abnormalities of the cardiac rhythm occur without warning. Despite the abundant information available from studies on the mechanisms of tachycardias, there was hardly a shred of objective data in 1989 to establish how the occurrence of symptomatic tachycardias was influenced by various mechanisms. In fact, there were very few objective data describing the occurrence of symptomatic tachycardia among the patients who were afflicted with various paroxysmal tachycardias. For example, were patients in normal sinus rhythm likely to remain free of their tachycardia for one day, one week, one month, or longer? The suddenness and apparent unpredictability of attacks of paroxysmal tachycardias have been substantial obstacles to quantitative description of their occurrence. These studies used careful documentation of spontaneous tachycardia to establish the epidemiology of symptomatic arrhythmias.