We evaluated the risk of developing clinically definite multiple sclerosis (CDMS) after an acute attack of isolated optic neuritis (ON) in 112 patients, in relation to demographic and paraclinical findings. Patients were examined by brain MRI, CSF analysis, and multiple evoked potentials (EPs); 10 were lost to follow-up, and the other 102 were enrolled in a prospective study (follow-up duration 6. 3 +/- 2.2 years). Of these, 37 (36.3%) developed CDMS after a mean interval of 2.3 +/- 1.6 years. The risk of developing CDMS was 13% after 2 years, 30% after 4, 37% after 6, and 42% after 8 and 10 years. Gender, age, and season of ON onset did not affect the risk. MS occurred in 37 of 71 patients (52.1%) with one MRI lesion or more; no patient with a normal MRI developed the disease. MS developed more frequently in patients with intrathecal IgG synthesis than in those without (43% vs. 28%), but the difference was not statistically significant. Multiple EPs showed a slight predictive value only including somatosensory EPs of the lower limb. Multiple sclerosis was mild in most cases (EDSS 2.2 +/- 1.9). The EDSS was less than 4 in 32 cases (86%), between 4 and 6 in 2 (5%), higher than 6.5 in 3 (8%).