The purpose of this study was to elucidate the clinical features of right ventricular (RV) dysplasia, a recently described clinical entity characterized by RV myopathic changes and ventricular tachycardia of left bundle branch block morphology. Five cases were reported, in which the diagnosis was established according to the criteria of Marcus. Case 1, a 33-year-old man, was referred to us for evaluation of his paroxysmal ventricular tachycardia of five years duration. Case 2, a 38-year-old man, was admitted because of shock caused by ventricular tachycardia. In both cases, the QRS configurations during ventricular tachycardia were those of the left bundle branch block pattern, and electrocardiograms during sinus rhythm showed T wave inversions in the right precordial leads and late ventricular potentials. Two-dimensional echocardiography, radionuclide angiography and contrast angiography disclosed RV dilatation and dysfunction with normal left ventricular (LV) function. The biopsied myocardium from the right and left ventricles in Case 2 revealed myocytolysis, a paucity of myofibrils and proliferation of collagen fibers. Case 3, a 73-year-old woman began to experience exertional dyspnea since 16 years of age. Her treatment consisted of bed rest, diuretics and digitalis. In December 1983, her New York Heart Association functional class was III, and physical examinations disclosed a Levine III/VI systolic murmur suggestive of tricuspid regurgitation, jugular vein dilatation, hepatomegaly, and pretibial edema. Electrocardiogram showed atrial fibrillation, incomplete right bundle branch block and T wave inversions in the precordial leads. Chest radiograph revealed marked cardiomegaly (cardiothoracic ratio of 92%). Echocardiography, radionuclide angiography and contrast angiography revealed marked RV dilatation, depressed ejection fraction (RV end-diastolic volume index of 342 ml/m2, and RV ejection fraction of 28%). Case 4, a 20-year-old man, was admitted with a chief complaint of palpitation. Case 5, a 19-year-old man, was referred to us for the evaluation of asymptomatic cardiomegaly, and his cardiothoracic ratio was 54%. A 12-lead electrocardiogram showed right bundle branch block and T wave inversions in the right precordial leads. Ambulatory electrocardiography revealed frequent premature ventricular complexes of the left bundle branch block pattern. Echocardiography, radionuclide angiography, and contrast angiography disclosed RV dilatation and dysfunction in both cases, and mild LV dilatation and dysfunction (LV end-diastolic volume index of 149 ml/m2, LV ejection fraction of 48%) in Case 4.(ABSTRACT TRUNCATED AT 400 WORDS)