Idiopathic acute transverse myelitis (ATM) is an immune-mediated inflammatory demyelinating disorder of the spinal cord with motor, sensory and autonomic involvement. Annual incidence is estimated at between 1/1,000,000 and 1/250,000 depending on the study. Onset may occur at any age and both sexes may be affected. The spinal cord inflammation is focal and the signs and symptoms are usually bilateral and depend on the extent and site of the lesion, with the thoracic spinal cord being the most common localization. Progression to nadir occurs between 4 hours and 21 days after onset. Motor involvement is characterized by limb weakness, stiffness and muscle spasms. If the upper spinal cord is involved then respiratory function may be impaired. Back pain, paraesthesia, numbness and neuropathic pain are common sensory manifestations. Uncomfortable band-like sensations around the torso and radicular pain have also been reported. Autonomic anomalies include sexual dysfunction, urinary urge/retention and bowel urgency/retention. Autonomic dysreflexia (ADR), resulting in rapid onset of hypertension and bradycardia, is a complication seen in patients with spinal cord lesions at T6 or above and usually with severe myelitis. By definition, the etiology of idiopathic ATM is unknown. A history of viral illness (usually upper respiratory infection) often precedes onset of symptoms by three weeks and idiopathic ATM is believed to be associated with a late immune response against a recent microbial infection that inadvertently targets the spinal cord. The diagnostic approach revolves around confirming the diagnosis of myelitis (MRI revealing transverse spinal cord lesions and swelling, with longitudinally extensive lesions in some cases), and excluding secondary causes (brain MRI, serology and analysis of cerebrospinal fluid to rule out secondary ATM; see this term), which may be associated with a relapsing disease course requiring preventative treatments. Acute compressive lesions (such as metastases and epidural abscess) and infarction of the spinal cord should also be included in the differential diagnosis. Acute treatment may include corticosteroid therapy and plasma exchange. The benefits of intravenous immunoglobulins and cyclophosphamide remain to be established. Long-term management is mainly symptomatic and should include rehabilitative therapy. The prognosis is variable and unpredictable. Recovery may begin between 2 and 12 weeks after the onset of symptoms. Full recovery (occurring in only a third of patients) may take years and permanent sequelae are frequent (moderate disability in one third of patients and severe disability in the remaining third).