Since the first description of multiple sclerosis (MS), it has been known that relapse of the disease may be triggered by febrile infections. 1 Although the prevention of febrile illness is therefore clearly advantageous for those with MS, there is considerable controversy as to whether vaccine should be administered to such individuals. A further subject of debate is the possibility that vaccination (against influenza, or hepatitis B) leads to the development of MS. The coincidence observed between the administration of hepatitis B vaccine and the onset or relapse of MS and other demyelinating diseases 2-5 has stimulated a number of studies of the possibility of their association. 6-12