Safety and Efficacy of Natalizumab in the Treatment of Multiple Sclerosis

Inclusion Criteria: * Diagnosis of MS, as defined by McDonald et al., criteria # 1-4 (McDonald et al., 2001) * Between the ages of 18 and 50, inclusive. * Baseline EDSS score between 0.0 and 5.0, inclusive. * Have experienced at least one relapse within the 12 months prior to randomization. * Cranial MRI scan demonstrating lesion(s) consistent with MS. * Have given written informed consent to participate in the study. Exclusion Criteria: * Primary progressive, secondary progressive, or progressive relapsing MS. * MS relapse has occurred,in the opinion of the investigator, within 50 days prior to randomization and/or the subject has not stabilized from a previous relapse. * A clinically significant infectious illness within 30 days prior to randomization. * History of, or abnormal laboratory results indicative of any significant cardiac, endocrinologic, hematologic, hepatic, immunologic, metabolic, urologic, pulmonary, gastrointestinal, dermatologic, psychiatric, renal and/or other major disease, that in the opinion of the investigator, would preclude the administration of a recombinant humanized antibody immunomodulating agent for 116 weeks. * History of severe allergic or anaphylactic reactions or known drug hypersensitivity. * Unable to perform the Timed 25-foot Walk, 9HPT, and PASAT 3. * Abnormal blood tests performed at the Screening Visit.