Abstract Neuromyelitis optica (NMO) is a rare demyelinating disease, affecting selectively the optic nerve and the spinal cord. It was previously considered to be a severe variant of multiple sclerosis (MS) due to the similar pathological features and its resemblance to optico-spinal, or Japanese, MS, typical of Asian populations. The finding that most NMO patients have auto-antibodies against aquaporin-4, a water channel particularly abundant on the astrocytes of the glia limitans, has allowed early diagnosis and specific treatment of these patients, and has greatly improved our knowledge of its pathogenesis. When laboratories worldwide can detect anti-aquaporin-4 auto-antibodies with comparable sensitivity and specificity, we will be able to have large multi-centric studies to define better the epidemiological, clinical and pathological features of patients and their responses to treatment.