The predictive value of CSF oligoclonal banding for the development of clinically definite MS (CDMS) within 5 years after optic neuritis was assessed in 76 patients enrolled in the Optic Neuritis Treatment Trial. The presence of oligoclonal bands was associated with the development of CDMS (p = 0.02). However, the results suggest that CSF analysis is only useful in the risk assessment of optic neuritis patients when brain MRI is normal and is not of predictive value when brain MRI lesions are present at the time of optic neuritis.