The absolute linoleic acid concentration in CSF was determined and the findings of MS patients (n = 10) and controls (n = 12) were compared. The linoleic acid content of control CSF (1.6 +/- 0.8 nMol/ml) is considerably lower than the corresponding serum value (2.5--4.1 muMol/ml). Although CSF from MS patients contains a significantly higher linoleic acid concentration than controls the close correlation between CSF linoleic acid and CSF albumin is maintained. The high CSF concentration of cholesterol esters rich in linoleic acid, which are abundant in serum but represent only traces in CNS lipids, points towards an impaired BBB function as the cause of CSF linoleic increase. We are able to show that both albumin and linoleic acid are suitable as "serum markers" and also as reference parameters for the overproportional IgG concentration in the CSF of MS patients. On the basis of these results it can be assumed that changes in CSF linoleic acid content are an expression of dysfunction of the blood-CSF barrier in MS and not, as had previously been postulated, the result of altered myelin metabolism.