We evaluated the risk of developing clinically definite multiple sclerosis (CDMS) after acute isolated optic neuritis in 102 patients in a follow-up study (duration 6.5 +/- 2.0 years). The probability of CDMS was 13% after 2 years, 30% after 4 years, 38% after 6 years, and 49% after 8 and 10 years. CDMS occurred in 42 (59%) of 71 patients with brain lesions detected with magnetic resonance imaging (MRI). No patient with normal MRI exam developed the disease. Patients with 3 or more MRI-detected lesions presented a shorter first interattack interval and a higher relapse rate compared to subjects with only 1 or 2 lesions. The predictive value of CSF examination and of evoked potentials was poor.