Alemtuzumab is a humanized monoclonal antibody targeting the surface molecule CD52, resulting in a rapid depletion of innate and adaptive immune cells. Infection rates in multiple sclerosis (MS) treatment trials were higher in alemtuzumab than in interferon beta-treated patients. We report two MS patients who developed cytomegalovirus disease within 1month after the first 5-day cycle of alemtuzumab. Upon identification and appropriate treatment of the infection, each recovered completely. Neurologists should be aware of this serious but treatable complication.