Alemtuzumab is a humanized monoclonal antibody against CD52, an antigen found on lymphocytes and monocytes. Pulsed administration causes prolonged T-cell depletion and has been shown to be effective in early relapsing-remitting multiple sclerosis, reducing relapse rate and risk of acquiring disability in comparison with the standard therapy IFN-b. Alemtuzumab is currently approved for the treatment of B-cell chronic lymphocytic leukemia but not for multiple sclerosis. The most significant complication of treatment is the late development of autoimmunity, which occurs in 30% of patients. Serious infections are rare. Phase III trials are ongoing and it is possible that alemtuzumab will have a place among the range of emerging disease-modifying therapies for multiple sclerosis.