Disturbed myocardial energy metabolism may occur in patients with primary hypertrophic cardiomyopathy (HCM). A noninvasive way to gain insight into cardiac energy metabolism is provided by in vivo 31 P nuclear magnetic resonance (NMR) spectroscopy. 31 P NMR spectroscopy with proton decoupling was performed in 13 patients aged 13–36 years with HCM on a 1.5 T Magnetom with a double resonant surface coil. A 2D chemical shift imaging (CSI) sequence in combination with slice selective excitation was used to acquire spectra of the anteroseptal region of the left ventricle (volume element: 38 mL). The chemical shifts of the phosphorus metabolites, intracellular pH i , and coupling constants J αβ and J γβ were calculated. Peak areas of 2,3-diphosphoglycerate (DPG), P i , and adenosine triphosphate (ATP) were determined and corrected for blood contamination, saturation, and differences in nuclear Overhauser enhancements (NOE). The maximum thickness of the interventricular septum (IVS max ) was determined from tomographic long-axis images and expressed as number of standard deviations above the mean of the normal population (Z score). The patients were then divided into 2 groups: 6 patients with moderate HCM (HCM m , Z score ≤5) and 7 patients with severe HCM (HCM s , Z score 5). No differences between both groups and a control group of healthy volunteers (n = 16) were found with respect to phosphocreatine (PCr)/γ-ATP ratio, pH i , or the coupling constants. Only the PCr/P i ratio differed significantly from the control group (HCM all , α <0.05, HCM s , α <0.02, 2-sided U test). The decrease of the PCr/Pi ratio in patients with HCM is probably caused by ischemically decreased oxygen supply in the severely hypertrophied myocardium.