Localized proton spectroscopy was performed in 15 patients with acute or chronic multiple sclerosis (MS). Some of the patients were investigated serially, being given a total of 22 spectroscopic investigations. Resonances corresponding to free lipids were observed in six plaques. This was distinctly seen in two plaques at Days 70 and 85 after the occurrence of the plaques. A lesser content of lipids in plaques was observed as early as Day 10 and as late as nearly 1 year after occurrence. The relative concentration of N-acetyl asparate (NAA) was significantly lower in patients than in controls, and the relative concentration of choline (Cho) was significantly higher in patients than in controls. These differences were most pronounced in older plaques. MR spectroscopic demonstration of lipids in a MS plaque probably reflects disintegration of myelin, and a decreased NAA/Cho ratio may be related either to gliosis or to axonal degeneration, which sometimes occurs in longstanding MS.