The aim of this study was to describe the demographic, clinical and laboratory features of idiopathic acute transverse myelitis (IATM). Patients with non-compressive ATM receiving care at Hospital da Lagoa, Rio de Janeiro (Brazil) between 2000 and 2008 were selected. Of the 70 cases of acute myelopathies, the idiopathic form was identified in 41 following exclusion of the cases associated with systemic lupus erythematosus (n = 1), Sjogren's syndrome (n = 1), herpes zoster (n = 1), cytomegalovirus in an HIV-positive patient (n = 1), Schistosoma mansoni (n = 1), actinic myelitis (n = 1), infectious myelitis of unknown etiology (n = 2) and those that, following the first attack of myelitis, converted to NMO (n = 19) or to clinically defined MS (n = 2). Of the 41 cases of IATM, the majority of patients were female (68.3%) and white (65.9%). Median age at first myelitis was 37.0 +/- 11.8 years. Over a median observation time of 36 months, 39.0% of patients remained monophasic, while recurrences occurred in 61.0% of cases. The number of ATM/patient ranged from one to seven. Among the recurrent cases the median time between the first and the second ATM was 12 months (range 1-150 months).The first myelitis was characterized mainly by partial myelitis with motor and sensorial dysfunction (63.4%). Complete and severe myelitis occurred more frequently among monophasic patients and partial myelitis with moderate dysfunction at onset in recurrent cases; however, over the long-term, dysfunction and disability were mild in both groups. Serial spine MRI confirmed spinal cord inflammation in 92.0% of cases and extensive spinal cord lesion was identified in 61.0%. Brain MRI was normal or not suggestive of MS in 94.4% of cases. CSF showed pleocytosis in 41.2%, increased IgG index in 24.0% and oligoclonal bands in 38.0% of 34 patients tested. Abnormal visual evoked potentials occurred in 11.5% of 26 patients. Positivity for anti-AQP4 was found in 23.5% of the 17 cases tested, suggesting limited forms of NMO. This study suggests some new aspects of the clinical course of IATM such as the high conversion rate to NMO, the predominance of women and a higher frequency of recurrent forms.