Acute transverse myelitis (ATM) is an etiologically heterogeneous syndrome with acute or subacute onset, in which inflammation of the spinal cord results in neurologic deficits, manifesting as weakness, sensory loss and autonomic dysfunction. It is frequently associated with infectious or systemic autoimmune diseases, but its etiology remains unknown in a substantial portion of cases, which are classified as idiopathic. Unifying diagnostic criteria for idiopathic and disease-associated ATM were proposed in 2002. Although they have been applied to a few cohorts of patients, the limited information provided in the relevant publications has not yet yielded many new insights on the clinical characteristics, disease course, and outcome of adult patients with idiopathic ATM compared to older studies that did not always distinguish between the various etiologies of ATM. There is, however, some new epidemiological data indicating that the incidence of idiopathic ATM is considerably higher, and the female preponderance greater, than previously recognized. In addition, new data on children with ATM show that the prognosis in pediatric patients is not always as benign as previous studies had indicated. The combination of ATM and optic neuritis characterize Devic's syndrome or neuromyelitis optica (NMO). A seminal discovery was the identification of an antibody that is a specific marker not only for NMO, but also of some of its characteristic manifestations in isolation, including longitudinally extensive TM. This has resulted in the proposal that all of the disorders that are associated with NMO-IgG positivity constitute part of an NMO spectrum of disorders. This antibody recognizes aquaporin-4, which represents the most abundant water channel of the central nervous system. There is growing evidence that the antibodies targeting this channel protein have pathogenic potential, thereby providing insights into the possible pathogenetic mechanisms of at least one type of ATM.