Microvolt T-wave alternans has been proposed as a new risk marker for ventricular arrhythmias. However, the clinical significance of T-wave alternans in patients with ventricular tachycardia (VT) originating from the right ventricle has been unknown. The study population consisted of 20 patients with arrhythmogenic right ventricular cardiomyopathy (ARVC) or idiopathic VT. T-wave alternans was measured during bicycle exercise testing using the CH 2000 system. Of the 7 patients with ARVC, 6 (86%) were positive for T-wave alternans. On the other hand, only 1 (8%) of 13 patients with idiopathic VT originating from the right-ventricular outflow tract was positive for T-wave alternans.