Arrhythmogenic right ventricular dysplasia (ARVD) is a recently identified clinical entity and its diagnosis rests on documentation of ventricular tachycardia (VT) of right ventricular origin and morphologic changes of the right ventricle. However, the diagnosis of ARVD is difficult noninvasively and often requires angiography. The usefulness of equilibrium radionuclide ventriculography as a noninvasive method for the diagnosis of ARVD has not been fully evaluated. We performed equilibrium radionuclide ventriculography in 7 patients with ARVD, 10 normal subjects and 9 patients with dilated cardiomyopathy (DCM). The right ventricular ejection fraction (EF) in patients with ARVD (25 +/- 8%, mean +/- SD) was lower (p less than 0.001) than that in normal subjects (56 +/- 7%) but was not significantly different from that in patients with DCM (32 +/- 10%). The left ventricular EF in patients with ARVD (57 +/- 12%) was lower (p less than 0.05) than that in normal subjects (70 +/- 7%), but higher (p less than 0.001) than that in patients with DCM (27 +/- 7%). These findings suggest that ARVD is a specific form of DCM which predominantly affects the right ventricle and that equilibrium radionuclide ventriculography may be a useful noninvasive method for the diagnosis of this disorder. In addition, we present a family in which 3 of 6 siblings were affected by ARVD, suggesting that some genetic factors may be involved in the etiology of this disorder.