The diagnostic significance of intrathecally synthesized IgG and virus-specific antibodies to measles, rubella and varicella-zoster (MRZ) in cerebrospinal fluid (CSF) remains controversial in cases of acute optic neuritis (AON). This study evaluates the prognostic value of baseline CSF and serum markers in AON, and correlates them with magnetic resonance imaging (MRI) and progression to multiple sclerosis (MS). Paired CSF and serum samples from 36 AON patients, 26 MS patients and 22 controls were analyzed for albumin, IgG, oligoclonal IgG (OI), MRZ antibodies, and blood-CSF barrier function; baseline MRI scanning of the head was also performed. The most sensitive parameter for detection of intrathecal inflammation in AON was OI (75%). Baseline MRI scans revealed abnormalities in 46% of the 28 patients with AON. Fifty percent of AON patients developed MS over the following 4 years. Ninety four percent of patients progressing to MS were positive for either OI, MRI or both. Of the AON patients initially positive for MRI and intrathecally-produced MRZ antibodies, 86% developed MS after 4 years. Only 17% of AON patients with negative results for OI and MRI developed MS. Six patients with abnormal OI but normal MRI progressed to MS. CSF and serum analyses, together with MRI, are the methods of choice for prognostic evaluation of patients with AON.