The clinical and paraclinical characteristics of acute transverse myelitis (ATM) were analyzed in 31 patients. In some patients there was clinical evidence of complete transection, in others of only partial lesions. Magnetic resonance imaging (MRI) in the acute phase in the first group was normal, but showed cord atrophy subsequently. It is probable that the clinical picture was due to parenchymatous neuronal lesions, analogous to those of axonal polyneuropathy. In the patients with incomplete transverse lesions, the most common finding was demyelination. In the patients with circumscribed demyelinating lesions, the symptoms and MRI were suggestive of clinically isolated syndromes (CIS) predictive of multiple sclerosis (MS). Extensive demyelination was indicative of acute disseminated encephalomyelitis (ADEM) due to hyperergic vasculopathy or various forms of chronic vasculitis. In two patients with variable clinical symptoms, a vascular malformation was the cause of the clinical presentation, and in one patient demyelination was due to the disc compression.