The main objective of this study is to evaluate the efficacy and tolerance of 2 years of treatment with cholecalciferol (vitamin D3) in patients with a clinically isolated syndrome at high risk for MS (CIS).

The secondary objectives of this study are:

A. evaluate clinical efficacy: delay to conversion; number of relapses/episodes per year B. evaluate efficacy in terms of resonance imaging parameters (cerebral/spinal MRI) C. evaluate efficacy in terms of slowing the progression of disability as measured by EDSS score and subscores D. measure and assess cognitive abilities (PASAT) E. evaluate changes in quality of life (EQ5D questionnaires, SF36, and TLS-TLS-QoL10 COPING10), fatigue questionnaire (FSMC) and anxiety / depression questionnaire (HADS) F. evaluate treatment tolerance G. to correlate changes in clinical and imaging parameters with the evolution of serum levels of 25(OH)D2 and 25(OH)D3 H. establish a biobank of DNA and RNA from all patients in the study and conduct analyses of gene polymorphisms involved in the metabolism of vitamin D and the HLA system based on the increased levels of vitamin D after supplementation I. establish a biobank of CSF, plasma, blood cells, serum and RNA samples for patients in selected centers for research on prognostic biomarkers of conversion J. establish a biobank consisting of plasma tubes collected for the determination of 25-hydroxy-vitamin D K. Estimate the rate of discordance between the conversion decision made by the study neurologist and the result of the MRI re-interpretation performed at the end of the study as well as the proportion of patients identified a posteriori as as erroneously included according to the centralized reading.

Inclusion Criteria:

• The patient must have given his/her informed and signed consent
• The patient must be insured or beneficiary of a health insurance plan
• The patient is available for 24 months of follow-up
• The patient has had a classic CIS with the past 90 days
• Reference cerebro-medullary MRI scheduled within the 90 days after the beginning of symptoms
• With MRI (cerebro ± medullary) showing demyelination according to spatial spread criteria by Swanton (2006):
• At least 1 lesion in at least 2 of the 4 following territories: (1) Peri-ventricular; (2) Juxta-cortical; (3) Sub-tentorial; (4) Medullary
• No other suspected pathology
• Vitamin D level in blood less than 100 nmol / l at the pre-inclusion visit
• Women of childbearing potential must use very effective contraception for the duration of the study. A very effective contraceptive method is defined as a method resulting in a low failure rate (that is to say less than 1% per year) when used consistently and correctly, such as implants, injectables, combined oral contraceptives, IUDs, sexual abstinence, or partner with a vasectomy.

Randomisation stratification criteria:

• The patient can also also meet the temporal dissemination criteria defined according to McDonald criteria 2010 (Polman et al., 2011), because this condition is currently not sufficient for prescribing a background treatment: Simultaneous presence of at least one asymptomatic lesion taking on contrast and at least one asymptomatic lesion not taking on contrast after injection of gadolinium

Exclusion Criteria:

• The patient is participating in another study (this criteria does not apply to the POLAR study (RCB 2011-A01269-32); patients included in this study may simultaneously participate in the POLAR study)
• The patient is in an exclusion period determined by a previous study
• The patient is under judicial protection, under tutorship or curatorship
• The patient refuses to sign the consent
• It is impossible to correctly inform the patient
• The patient is pregnant, parturient, or breastfeeding
• Major medical or psychiatric illness that, according to the investigator, would result in the patient running an unnecessary risk or that could affect compliance with the study protocol
• Vitamin D insufficiency linked to currently active digestive or more general diseases (celiac disease, inflammatory bowel disease, intestinal bypass, short bowel syndrome, cirrhosis, nephrotic syndrome, hyperthyroidism, rickets, hypoparathyroidism, cancer, granulomatous diseases and lymphomas)
• Moderate or severe renal insufficiency (creatinine clearance less than 60 ml / min)
• Epilepsy not adequately controlled by treatment
• Any illness requiring chronic treatment with corticosteroids
• Patient with osteoporosis or history of osteopenia
• Pathology requiring calcium intakes greater than 1 gram per day
• Current or past history of hypercalcemia
• Medications that affect the metabolism of vitamin D other than corticosteroids; e.g. anticonvulsants [phenobarbital, primidone, phenytoin] rifampicin, isoniazid, ketoconazole, 5-FU and leucovorin, thiazide diuretics.
• Situations accompanied by increased vulnerability to hypercalcemia, e.g. arrhythmia or known heart disease, treatment with digitalis, and subjects with nephrolithiasis.
• Contraindications to vitamin D3 as mentioned in the documentation for UVEDOSE
• Known hypersensitivity to gadolinium and / or known inability to undergo an MRI (pacemaker, osteosynthesis material, intraocular metal splinter, etc ....).