Magnetic resonance imaging (MRI) and proton magnetic resonance spectroscopy (MRS) were evaluated for differentiating metastatic brain tumors, radiation necroses, and brain abscesses. Twelve histologically verified lesions in 12 patients were studied using preoperative MRI and proton MRS. The signal intensities of four major metabolites, N-acetyl aspartate (NAA), choline-containing compounds (Cho), creatine and phosphocreatine (Cr), and lactate (Lac), were observed over the region of interest. Metastatic brain tumors showed a decrease in NAA/Cr and an increase in Cho/Cr ratios. Radiation necroses showed a decrease in NAA/Cr and no change in Cho/Cr ratios. Brain abscesses showed an increase in Lac/Cr ratio. Correlation with histopathologic findings showed that a high Cho signal was suggestive of a metastatic brain tumor. Lac signals were observed in brain abscesses, presumably reflecting the anerobic glycolysis of living cells. Although more cases and studies are necessary, metabolic information provided by proton MRS combined with MRI is useful for differentiating among metastatic brain tumors, radiation necroses, and brain abscesses.