We reviewed the magnetic resonance imaging (MRI) database of the Dent Neurologic Institute to study the abnormal findings in myelitis. We identified 22 patients, and compared non-MS-related acute transverse myelitis (ATM, n = 9), to myelitis associated with multiple sclerosis (MS-myelitis, n = 13). The ATM patients were significantly older than MS patients at the time of the myelitis diagnosis (mean age 46 vs 35, p < 0.05). ATM appeared as a "longitudinal myelitis", with fusiform cord expansion on T1-weighted images and intramedullary increased signal on T2-weighted images, each involving multiple spinal levels (mean = 7-8). However, MS-myelitis lesions appeared focal, involving significantly fewer spinal levels (mean = 1-2, p < 0.001), although the lesions were equally likely to expand the cord. Four (42%) of the ATM lesions showed abnormal, variable enhancement, whereas none of the MS myelitis lesions enhanced. Cranial MRI was more likely to be normal in ATM (78%) than in MS-myelitis patients (15%, p < 0.001). Although readily distinguishable from lesions due to MS, the various etiologies for ATM, including post-infectious (n = 2), post-vaccination (n = 3), and idiopathic (n = 4) were indistinguishable on MRI. The MRI findings of an extensively lesioned, swollen cord, suspicious for an intramedullary tumor and providing a temptation for a biopsy, may reflect a non-neoplastic inflammatory disorder.