The lipid resonances detected in proton MR spectra of brain tumors were evaluated from the diagnostic point of view. Seventy five non-radiated brain tumors were examined in vivo and lipids were detected in 29% of anaplastic astrocytomas grade III, 60% of glioblastomas multiforme and 50% of metastatic tumors. Lipids were also detected in all brain abscesses and an epidermoid cyst. No lipids were detected in benign tumors (low grade and pilocytic astrocytomas, oligodendrogliomas, meningiomas, pituitary adenomas etc.). Thus, the presence of lipids in the MR spectra is indicative of malignancy (with a few exceptions).