Background/Purpose: Optic neuritis (ON) is a rare disease that may remain a single episode or transform into MS. OCBs and spinal MRI lesions have already been identified as prognostic factors. Aim: Our aim was to evaluate if the presence of more than one elevated antibody index of measles, rubella, and/or varicella-zoster (MRZ) is an indicator of risk of conversion. Methods: In total, 228 patients diagnosed with ON between 1990 and 2013 were included in this retrospective study. All children had a data set consisting of age, sex, ON type, MRI, and detailed CSF studies, including the presence of OCBs and MRZ reactions and a follow-up of at least 1.5 years. Children were then divided into two groups: those who developed MS according to the McDonald criteria 2010 (n = 92) and those who did not (n = 136). Binary logistic regression analysis was used to assess the relationship between the different prognostic factors and conversion to MS. Positive (PPV) and negative predictive values were calculated. Results: Binary logistic regression analysis revealed that an MS-like MRI (p < 0.001), positive OCBs (p = 0.002), and a positive MRZ reaction (p < 0.001) were significant prognostic factors for conversion to MS after ON. Calculated PPVs showed a positive MRZ reaction alone to already be a good predictor (PPV 0.90 (95%CI: 0.82 to 0.95), p < 0.001). The best prediction was possible with a combination of cMRI, the presence of OCBs, and a positive MRZ reaction (PPV 1.00 (0.93 to 1.00), p < 0.001). Conclusions: Our findings show that a positive MRZ reaction alone already has a high predictive value for future conversion to MS and should be included in the workup of a child with an initial demyelinating event.