Accumulation of triglycerides (TG) in heart tissue has been associated with changes in left ventricular function. Proton magnetic resonance spectroscopy is currently the only noninvasive in vivo method to measure myocardial triglycerides content. The primary aim of this study was to determine if these in vivo measurements are specific to myocardial triglycerides in human subjects. Thus, in vivo proton magnetic resonance spectroscopy measurements were conducted on orthotopic heart transplant patients (n = 8) immediately before they underwent routine biopsies and ex vivo measurements were made on the endomyocardial biopsy samples. The correlation coefficient between the two measurements was 0.97, with P < 0.005, demonstrating for the first time the specificity of the in vivo measurement in human heart. From accompanying reliability experiments, the standardized typical error for the in vivo proton magnetic resonance spectroscopy method was estimated to be 7.0%, with a 95% confidence interval from 5.5 to 9.4%. These results suggest that proton magnetic resonance spectroscopy provides a specific and reliable measurement of myocardial triglycerides content and is suitable for routine studies. Magn Reson Med, 2011.