Multiple sclerosis (MS) is a chronic inflammatory demyelinating disease of the CNS. Approximately 2 million people worldwide have MS, with females outnumbering males 2:1. Because of its high prevalence, MS is the leading cause of nontraumatic neurologic disability in young adults in the United States and Europe. Axon loss is the major cause of irreversible disability in patients with MS. Axon damage, including transection of the axon, begins early in MS and correlates with inflammatory activity. Several mechanisms lead to axon loss, including inflammatory secretions, loss of myelin-derived support, disruption of axonal ion concentrations, energy failure, and Ca(2+) accumulation. Therapeutic interventions directed toward each of these mechanisms need to be tested for their efficacy in enhancing axon survival and, ultimately, their ability to delay progression of neurologic disability in patients with MS.