Myocardial high-energy phosphate metabolism in patients with dilated cardiomyopathy (DCM) of ischemic or idiopathic etiology was assessed at rest by one-dimensional phase-encoded 31 P-nuclear magnetic resonance (NMR) spectroscopy studies performed in conjunction with 1 H imaging in 20 patients with DCM and in 12 normal volunteers. The messured values of anterior myocardial phosphocreatine/β-adenosine triphosphate (PCr β-ATP), corrected for partial saturation and contamination of the spectra by blood metabolites, averaged 1.80±0.06 (mean±SE) in normal volunteers and 1.46±0.07 in the patients overall, a highly significant (p< 0.001) decrease. In patients with DCM accompanied by coronary artery disease (n= 9), the PCr β-ATP ratio averaged 1.53±0.07, while in those with DCM alone it was 1.41±0.12 (n= 11), a value that was not significantly different. There was no significant correlation (r= 0.34) between myocardial PCr ATP ratio and left ventricular ejection fraction in patients. These studies demonstrate that myocardial PCr ATP ratios are reduced at rest in human ischemic and idiopathic dilated cardiomyopathy.