The lipid composition of CSF, serum and lymphocytes from patients without demyelinating disease (controls) as well as from patients with acute and chronic MS is analyzed. Individual lipid fractions are isolated by TLC and their fatty acid composition determined. Lipid and fatty acid composition of normal CSF resembles the results found in lymphocytes and it is deduced that CSF lipids are derived mainly from lymphocytes rather than white matter (myelin) or serum. There is an increase of CSF sphingomyelin in patients with acute MS (showing pleocytosis) which is apparently derived from disintegrated lymphocytes; there is also an increase of linoleic acid concentration which could come from serum because of dysfunction of the blood-brain barrier. The role of the CNS in contributing to CSF lipids is considered negligible both in controls and in patients with demyelination.