 Alemtuzumab treatment was on a fixed schedule of two treatment courses a year apart for participants who received Rebif in one of the prior Genzyme-sponsored studies of alemtuzumab or on an as needed schedule e.g. due to documented evidence of resumed Multiple Sclerosis MS activity for participants who had already completed a fixed schedule of treatment with alemtuzumab in one of the prior Genzyme-sponsored studies. There was no comparison treatment in this study. All participants were required to return to their study site every 3 months for neurologic and other assessments. In addition, safety-related laboratory tests and surveys were performed at least monthly. Participation in the extension study was at least 48 months from enrollment. Study duration could be extended to allow participants to remain in the study until a follow-up study was available in their country or through month 60 month 72 in USA, whichever occurred first. 