The CSF level of the isoprostane 8-epi-prostaglandin (PG)-F-2 alpha (a reliable marker of oxidative stress in vivo) was three times higher in subjects with definite MS than in a benchmark group of subjects with other neurologic diseases. This increase was not correlated with that of PGE, levels, measured as an index of cyclooxygenase activity, and was much lower in steroid-treated patients. The levels of 8-epi-PGF(2 alpha) were moderately correlated with the degree of disability.