Optic neuritis (ON) is highly associated with multiple sclerosis (MS) and conversion rate has been reported to be around 50% over a period of 15 years. We revised the risk factors for developing MS after an attack of ON, in western Turkey. One hundred and twenty-four patients with acute unilateral ON, who were on follow-up for at least 5 years (range 62 to 180 months, mean 85 months) were enrolled in the study. Gender, age at onset, seasonal occurrence, severity of visual loss, presence of pain, optic disc appearance, treatment, degree of recovery, presence or absence of lesions on magnetic resonance imaging (MRI) during the initial ON attack and recurrence were considered. Of the 124 patients, 89 (71.8%) were women, 35 (28.2%) were men, with a mean age of 28.7 +/- 8.6 years at the time of the initial ON attack. During follow-up, 70 patients (56.5%) developed MS within 17.14 months, ranging from 1 to 125 months. Female gender (p = 0.021), experiencing the attack before 40 years of age (p = 0.02), normal-appearing optic discs (p = 0.005), presence of MRI lesions (p < 0.001), and recurring ON attacks (p = 0.014) were significantly associated with a high risk of developing MS. The period for development of MS was significantly shorter in patients with MRI lesions (p < 0.001). Optic neuritis patients with brain MRI findings showing the morphological evidence of disseminated disease can be considered to have MS at the time of the initial ON attack, and disease-modifying treatments can be initiated.