In order to assess the degree and type of cardiac involvement in patients with sustained hypereosinophilia, we studied by two-dimensional, M-mode echocardiography and Doppler (4 cases) 20 patients distributed into 2 groups. Group I: 10 patients with the idiopathic hypereosinophilic syndrome. Group II: 10 patients with secondary hypereosinophilia. In group I, 6 patients (60%) had echocardiographic abnormalities consistent with the endomyocardial disease: four apical obliteration of right ventricle, three apical obliteration of left ventricle, three endocardial thickening of the left ventricle, three endocardial thickening of the left ventricle posterior wall, one endocardial thickening of the right ventricle free wall, three subvalvular mitral thickening, three subvalvular tricuspid thickening, two pericardial effusion and two protodiastolic septal notch. These corresponded to 2 cases of endomyocardial fibrosis and restriction, as shown by pathological and hemodynamic study. In only 1 patient from group II echocardiographic abnormalities consistent with right apical occupation and tricuspid subvalvular thickening, with mild regurgitation detected by Doppler, were found. It was concluded that echocardiographic abnormalities are common in patients with idiopathic hypereosinophilic syndrome, even in the absence of clinical features. The development of echocardiographic abnormalities in patients with sustained secondary hypereosinophilia is exceptional and is probably related to duration of eosinophilia. Therefore, we think that echo-Doppler is a fundamental investigation for the diagnosis and follow-up of these patients.