To define the clinical and paraclinical profile of optic neuritis (ON), patients with a suspicion of ON among a population of 1.5 million were examined over 2.5 years. A diagnosis of monosymptomatic ON was established in 74 patients. Cerebrospinal fluid (CSF) studies in 73 patients revealed oligoclonal IgG bands in 67% and 32 of 60 patients examined (53%) had three or more high signal lesions on magnetic resonance imaging (MRI). A strong correlation was found between oligoclonal bands and abnormal MRI. In 52 patients, two or more CSF examinations revealed strong variations in individual patients for mononuclear cell count and IgG index. In contrast, of 39 patients with oligoclonal bands in the first sample, none showed the disappearance of bands, and of 13 patients initially negative for bands, only one developed bands. There was no correlation between exacerbation or remission and CSF findings. During a short-term follow-up of 6-40 months, 19 patients converted to MS.