In order to investigate the anatomic substrate of 'idiopathic' ventricular tachycardia (VT) 10 patients with chronic recurrent VT and no apparent sign of heart disease underwent an echocardiographic, haemodynamic and histologic study (5 males, 5 females: mean age = 40 +/- 11 years). In the patients with a left bundle branch block morphology of VT (7 cases), four showed findings compatible with an arrhythmogenic right ventricular dysplasia or a right ventricular cardiomyopathy. In the other three all examinations were normal with the exception of endomyocardial biopsy, which showed slight non specific changes in two. Of the remaining 3 cases (characterized by a right bundle branch block morphology of VT or by the presence of polymorphic VT) one had histologic evidence of myocarditis while another developed dilated cardiomyopathy. Macroscopic and/or microscopic ventricular abnormalities are frequently found in patients with VT which appears idiopathic. In these cases myocardial disease is frequently progressive, despite optimal control of VT.