The purpose of this report is to describe the clinical and electrophysiologic findings for patients with fascicular tachycardia (FT). Eight patients with FT, defined as tachycardia with HV interval during tachycardia less than that of HV of conducted impulses, were studied. ECG findings during FT included right bundle block and superior axis (four patients), right bundle branch block and inferior axis (one patient), and nonspecific intraventricular conduction delay (three patients). In three patients, entrainment as well as the ability to initiate and terminate the tachycardia favored a reentrant mechanism. In others, tachycardia initiation only over a critical range of paced cycle lengths and the incessant nature of the tachycardia or the presence of other atrial or ventricular foci favored a mechanism of either abnormal automaticity or triggered rhythms. Catheter ablation was successful in two of five patients in whom it was attempted. In conclusion, the ECG expression of FT is variable, depending on the site of origin of the arrhythmia in the ventricular specialized conduction system. Similarly, a variety of mechanisms and different foci may be associated with FT. Selected individuals may respond to catheter ablative therapy.