The accuracy of a newly developed nuclear magnetic resonance-computed tomography (NMR-CT) technique in diagnosing idiopathic cardiomyopathy was assessed and compared with other procedures such as echocardiography, coronary angiography, left ventriculography, myocardial biopsy, and electrocardiography. In case 1, the NMR-CT clearly revealed thickening of the lateral ventricular free wall and ventricular septum, which strongly suggested hypertrophic cardiomyopathy. Catheterization showed a pressure gradient of 54 mm Hg and this patient was diagnosed as having hypertrophic obstructive cardiomyopathy. In case 2, the NMR-CT showed dilatation of the ventricular cavity indicative of dilated cardiomyopathy. This was confirmed by echocardiography, which revealed the enlarged cavity of the ventricle and poor movement. In case 3, the patient had marked hypertension; the cardiac silhouette was enlarged, but the NMR-CT revealed that the ventricular free wall and septum were of normal thickness. This study shows that NMR-CT is of value in the differential diagnosis of idiopathic cardiomyopathy.