Background Previous ³¹P magnetic resonance spectroscopy (MRS) studies demonstrated that the myocardial phosphocreatine-to-ATP ratio offered important information concerning the degree of dysfunction and prognosis in patients with cardiomyopathy. In the present study, we investigated total creatine (CR) levels in various diseased hearts using ¹H MRS. Methods and Results Fourteeen patients with the following conditions were examined: cardiac amyloidosis (n=2); hypertensive heart disease (4); valvular disease (2); hypertrophic cardiomyopathy (2); dilated cardiomyopathy (2); restrictive cardiomyopathy (1); and post-operative atrial septal defect (1). Myocardial CR was measured using ¹H MRS with point-resolved spectroscopy localization. Overall, myocardial CR levels in diseased hearts were significantly lower than those in the control group [16.5±6.0 (n=14) vs 27.1±3.2 μmol/g (n=10), p<0.001]. There was a positive correlation between myocardial CR and left ventricular ejection fraction (42.9±13.8%, range 19.5-69.1%) despite the different mechanisms of cardiac dysfunction (r=0.60, p<0.05). Myocardial CR levels in patients who were hospitalized due to heart failure within 1 year were significantly lower than those in other patients [11.3±1.0 (n=4) vs 18.6±5.9 μmol/g (n=10), p<0.05]. Conclusions Noninvasive measurement of myocardial CR using ¹H MRS may be valuable in the assessment of disease severity and prediction of clinical course in various forms of heart disease. (Circ J 2005; 69: 711 - 716)