The precise pathogenesis of multiple sclerosis is unknown. The assumption of a primary immunopathogenesis of the disease is seriously flawed and has failed to deliver an effective therapy for most patients. The progressive degeneration of grey and white matter is integral to the natural history of the disease and is reflected in the atrophy of brain and spinal cord. Demyelination is an essential component of this primary neurodegenerative process rather than the target of a systemic immune response. The primary pathology of multiple sclerosis is a process of neurodegeneration based on the integrity of the blood-brain barrier. Primary progressive multiple sclerosis is the prototype neurodegenerative disease, and the relapsing-remitting form in younger population represents the modifying effect of steroids (vitamin D, sex and stress hormones) on metabolic functions of the central nervous system.