Using image-guided volume-selection techniques, in vivo phosphorus-31 magnetic resonance (MR) spectroscopic profiles were obtained from 12 patients with malignant breast tumors, six patients with benign breast tumors, and nine volunteers with no underlying pathologic condition. Phosphatic metabolites identified in the spectral profiles included the phosphomonoesters (PME), inorganic phosphate (Pi), phosphodiesters (PDE), phosphorylated glycans (PG), phosphocreatine (PCr) and adenosine triphosphate (ATP). Based on the results of previous high-resolution ex vivo 31P MR spectroscopic analyses of breast tissues, the resonance of PG was identified in malignant and benign breast tumors. Malignant tumors were found to have a significantly (P less than .05) lower concentration of (PME + Pi) than normal breast parenchyma, and were distinguishable from both benign tumors and normal breast parenchymal tissue by significantly (P less than .01) elevated levels of (PDE + PG). 31P MR spectroscopy is the first technique potentially capable of differentiating among malignant breast tumors, benign breast tumors, and normal breast parenchymal tissues based on their in vivo phosphatic metabolic profiles.