Multiple sclerosis (MS) presents with optic neuritis (ON) in 20 % of cases and 50 % of ON patients develop MS within 15 years. In this study, we evaluated the preventive effects of vitamin D3 administration on the conversion of ON to MS (primary outcome) and on the MRI lesions (secondary outcome) of ON patients with low serum 25 (OH) D levels. Thirty ON patients (15 in each of 2 groups, aged 20-40 years) with serum 25 (OH) D levels of less than 30 ng/ml were enrolled in a double blind, randomized, parallel-group trial. The treatment group (cases) received 50,000 IU of vitamin D3 weekly for 12 months and the control group (controls) received a placebo weekly for 12 months. Finally, the subsequent relapse rate and changes in MRI plaques were compared between the two groups. Risk reduction was 68.4 % for the primary outcome in the treatment group (relative risk = 0.316, p = 0.007). After 12 months, patients in the treatment group had a significantly lower incidence rate of cortical, juxtacortical, corpus callosal, new T2, new gadolinium-enhancing lesions and black holes. The mean number of total plaques showed a marginally significant decrease in the group receiving vitamin D3 supplementation as compared with the placebo group (p = 0.092). Administration of vitamin D3 supplements to ON patients with low serum vitamin 25 (OH) D levels may delay the onset of a second clinical attack and the subsequent conversion to MS.