The main aetiologies of acute myelopathy (AM) are: multiple sclerosis, systemic disease (SD), spinal cord infarct (SCI), parainfectious myelopathy (PIM) and delayed radiation myelopathy (DRM). Although a large amount of data have been published for each individual aetiology, comparison studies are scarce. The aim of this study was to assess the various aetiological and outcome profiles of AM. We studied 79 cases: 34 (43%) in multiple sclerosis; 13 (16.5%) in SD; 11 (14%) in SCI; five (6%) in PIM; and three (4%) in DRM. Myelopathies were of unknown origin in 13 (16.5%) patients. We evaluated clinical, spinal cord and brain MRI, CSF and evoked potentials data at admission, MRI outcome at 6 months and clinical outcome at 12 months. A statistical comparison of clinical, laboratory and outcome data was only performed between multiple sclerosis, SD and SCI patients due to the small number of cases in the other groups. A motor deficit was more frequent in SD and SCI than in multiple sclerosis where initial symptoms were predominantly sensory (P < 0.001). Spinal cord MRI showed lateral or posterior lesions of less than two vertebral levels in multiple sclerosis, in contrast to SD and SCI, where lesions involved more vertebral levels and were centromedullar (P < 0.001). Brain MRI was most frequently abnormal in multiple sclerosis (68%), but was also abnormal in 31% of SD patients (P < 0.05). Oligoclonal bands in CSF were more frequent in multiple sclerosis than in SD (P < 0.001) and were never found in SCI. Clinical outcome at 12 months was good in 88% of multiple sclerosis cases, and poor or fair in 91% of SCI and 77% of SD. Aetiologies of AM may be differentiated on the basis of clinical, spinal cord and brain MRI, CSF and outcome data, and allow a probable diagnosis to be made in previously undetermined cases. These findings may have therapeutic implications for cases with a questionable diagnosis.