A demyelinating disease is any disease of the nervous system in which the myelin sheath of neurons is damaged. This impairs the conduction of signals in the affected nerves, causing impairment in sensation, movement, cognition, or other functions depending on which nerves are involved.

The term describes the effect of the disease, rather than its cause; some demyelinating diseases are caused by infectious agents, some by autoimmune reactions, and some by unknown factors. Organo-phosphates, a class of chemicals which are the active ingredients in commerical insectacides such as sheep dip, weed-killers, and flea treatment preparations for pets, etc, will also demyelinate nerves.

Demyelinating diseases include multiple sclerosis, transverse myelitis, Guillain-Barré syndrome, and progressive multifocal leukoencephalopathy.