Neuromyelitis optica (NMO), also known as Devic syndrome, is defined clinically by severe inflammatory lesions in the optic nerves and spinal cord. Though it was lumped with multiple sclerosis for many years, it is now a clearly defined disease entity by clinical, imaging, and laboratory criteria. The main laboratory markers are high levels of antibodies to aquaporin 4, which is localized in the most affected areas. This disease may be the first of many to be separated out of the group of multiple sclerosis syndromes.