The primary objective of this initiative is to assess the effectiveness of subcutaneous (sc) interferon (IFN) beta - 1a, (Rebif®), versus No Treatment in delaying the conversion to Clinically Definite Multiple Sclerosis (CDMS) - as defined by the occurrence of a second exacerbation - over 96 weeks in subjects that present with Clinically Isolated Syndrome (CIS) accompanied by an abnormal magnetic resonance imaging (MRI). The secondary objectives are to:

• Assess the effectiveness of sc IFN beta - 1a (Rebif®) therapy in reducing the proportion of patients with CIS converting to CDMS
• Assess the safety of sc IFN beta - 1a (Rebif®) in the patients with CIS

Inclusion Criteria:

• Subject must have experienced a first clinical episode suggestive of demyelinating disease
• Subject must present with an abnormal MRI displaying at least 3 T2 weighted hyperintense lesions typical of multiple sclerosis (MS)
• Subject must be greater than or equal to 18 years old
• Subject must have had onset of the clinical attack within the last 120 days
• Subject must give written informed consent
• Female subjects must be neither pregnant nor breast feeding, and must not be of child-bearing potential as defined by either:
• Being post-menopausal or surgically sterile
• Using hormonal contraceptive, intra-uterine device, diaphragm with spermicide or condom with spermicide for the duration of the study

Subjects electing treatment:

• Subject must be eligible for Interferon-beta 1-a therapy

Exclusion Criteria:

• Subject has evidence of other neurological diseases that could explain his/her symptomatology
• Subject is pregnant or in lactation
• Subject suffers from an intercurrent autoimmune disease
• Subject suffers from major medical or psychiatric illness that in the opinion of the investigator creates undue risk to the subject or could affect compliance with the procedures required by this study
• Subject has received immunomodulatory or immunosuppressive therapy (including but not limited to cyclophosphamide, cyclosporine, methotrexate, azathioprine, linomide, mitoxantrone, teriflunomide, natalizumab, laquinimod, campath), within 12 months of study day 1

Subjects electing treatment:

• Subject has inadequate liver function, defined by total bilirubin, aspartate transaminase (AST), alanine aminotransferase (ALT), or alkaline phosphatase > 2.5 times the upper limit of normal values
• Subject has inadequate bone marrow reserve, defined as white blood cell count less than 0.5 times the lower limit of normal
• Subject has a known allergy to IFN or any of the excipients of the drug product

Up to Week 96

AE: any untoward medical occurrence in form of signs, symptoms, abnormal laboratory findings, or diseases that emerges or worsens relative to baseline during a clinical study with an Investigational Medicinal Product (IMP), regardless of causal relationship and even if no IMP has been administered. AE were not captured for "No Treatment" group.