BackgroundMultiple sclerosis (MS) is characterized by a polyspecific B-cell response to neurotropic viruses such as measles, rubella and varicella zoster, with the corresponding antibodies measurable in CSF as the so-called “MRZ reaction” (MRZR). We aimed to evaluate the relevance of MRZR to predict conversion of patients with clinically isolated syndrome (CIS) to MS, and to compare it to oligoclonal bands (OCB) and MRI.Methodology/Principal FindingsMRZR was determined in a prospective study over 2 years including 40 patients that remained CIS over follow-up (CIS-CIS) and 49 patients that developed MS (CIS-RRMS) using ELISA. Using logistic regression, a score (MRZS) balancing the predictive value of the antibody indices included in MRZR was defined (9 points measles, 8 points rubella, 1 point varicella zoster, cutpoint: sum of scores greater 10).MRZR and MRZS were significantly more frequent in CIS-RRMS as compared to CIS-CIS (p = 0.04 and p = 0.02). MRZS showed the best positive predictive value (PPV) of all parameters investigated (79%, 95%-CI: 54–94%), which could be further increased by combination with MRI (91%, 95%-CI: 59–99%).Conclusions/SignificanceOur data indicate the relevance of MRZR to predict conversion to MS. It furthermore shows the importance of weighting the different antibody indices included in MRZR and suggest that patients with positive MRZR are candidates for an early begin of immunomodulatory therapy.