Our series is the largest to date on the use of adenosine triphosphate in patients with idiopathic VT. By utilizing the drug as a diagnostic tool to differentiate between reentry and triggered automaticity, we sought to identify the mechanisms prevalent in our population of patients, and observed a very good correlation between the mechanism of VT and its anatomic origin. The drug reproducibly terminated tachycardia in all patients except 1 with LBBB morphologic VT, an-d in no patient with RBBB morphologic VT. Our results strongly support the current view that VT of left ventricular inferoapical origin is primarily a reentry phenomenon, whereas VT of right ventricular outflow tract origin may be a heterogenous group in which the predominant mechanism is triggered automaticity. © 1994.