To demonstrate the usefulness of in vivo 31P nuclear magnetic resonance spectroscopy (MRS) in the diagnosis of ischemic heart disease, major findings from three clinical cardiac MRS investigations performed at our institute are summarized. The first study investigated whether 31P MRS with handgrip exercise testing could detect myocardial ischemia demonstrated by exercise 201Tl scintigraphy. Contrary to findings in normal subjects or patients with fixed thallium defects, the ratio of phosphocreatine (PCr) to ATP decreased significantly during exercise in patients with reversible thallium defects. In the second study, PCr and ATP content was measured by 31P MRS and compared in human myocardium with reversible ischemia or scar diagnosed by exercise thallium scintigraphy. Although the PCr content decreased in patients with either reversible or fixed thallium defects, the ATP content decreased only in the latter group. In the third study, postischemic myocardium with chronic mechanical dysfunction that exhibits recovery after revascularization in left ventriculography was metabolically characterized using quantitative cardiac 31P MRS. Postischemic myocardium with reversible mechanical dysfunction demonstrated reduced PCr but normal ATP content. These results suggest that 31P MRS is a clinically important method both for the detection of myocardial ischemia and in the evaluation of myocardial viability.