Fifteen institutions cooperated to examine the ability of 1H MRS to characterize metabolism in vivo in 102 primary brain tumors. Spectra were acquired from single 8 cc voxels in the tumor using a spin-echo method with an echo time of 135 ms. The most intense lipid signal was that of fatty acyl methylene protons at 1.3 ppm. Intratumoral lipid signals were not detected in oligodendrogliomas, ependymomas, or meningiomas and were evident in only 1 of 6 primitive neuroectodermal tumors. Among 75 astrocytic tumors, lipid signals occurred in 16% of low grade astrocytomas (AS), 36% of anaplastic astrocytomas (AA), and 44% of glioblastomas (GB). The amount of lipids, expressed as a ratio of the intensity of the methylene signal to that of choline, increased progressively with histopathological grade (p = 0.05). Greater amounts of mobile lipids in vivo in GB, which usually contain necrosis, is in accord with the recently discovered correlation between the fraction of microscopic necrosis and the intensity of the mobile lipid signal observed ex vivo using 1H MRS. The observation of lipid signals in AA, which do not contain necrosis, suggests that mobile fatty acids may appear at a stage of metabolic insult that precedes microscopic signs of cell death, and raises the possibility of an independent correlation between 1H MRS-detectable lipids and prognosis in patients with astrocytic tumors.