We examined untreated 60 patients with acute monosymptomatic optic neuritis (ON). Patients examined early after onset showed increased expression of HLA-DR and CD45R0 on CD4 and CD8 T cells. Expression of HLA-DR on CD4 T cells was higher in patients without IgG oligoclonal bands. Expression of HLA-DR on CD4 and CD8 T cells correlated negatively with measures of disease activity and positively with measures of good visual function, and expression of CD45R0 on CD4 T cells correlated negatively with measures of disease activity. We hypothesize that HLA-DR expression may characterize a protective T-cell subset in ON