Baseline electrophysiologic methods, including intracardiac recording and programmed electrical stimulation, were used to determine the mechanism of paroxysmal supraventricular tachycardia. All patients had antiarrhythmic medications stopped. In most cases, the diagnosis of atrial fibrillation was established by scalar electrocardiographic criteria. At the time of entry into follow-up each patient was given a cardiobeeper and instructed to record and transmit any symptomatic arrhythmia when it occured. Descriptive information about each patient was entered into a baseline data file which included information on the time interval between attacks, age, sex, mechanism of arrhythmia, types of associated heart diseases, ECG data during sinus rhythm, and date and time of call. The purpose of the outpatient follow-up was to obtain objective documentation of spontaneously occurring, symptomatic tachycardia for quantitative analyses. Holter monitoring was used in patients with paroxysmal tachycardias to establish that asymptomatic tachycardia did not occur so often that it constituted an important, unrecognized feature of these clinical conditions.