Spinal cord cavitation is a frequent finding in optic neuromyelitis (Devic's syndrome) (DS) but it is also, although rarely, observed in patients with multiple sclerosis (MS). The objective of our study was to compare the MRI characteristics of the syringomyelic cavities in 6 patients with DS and 3 patients with MS. All the patients with DS had a relapsing clinical form with normal brain MRI. Spinal MRI revealed unenhanced central cavities which extended more than 3 vertebral bodies and remained unchanged in follow-up studies. Two patients presented multiple cavities.MS patients suffered a relapsing remitting form of the disease, they all had hyperintense T2 enhancing lesions on their spinal MRI. Moreover spinal MRI also revealed non communicating cavities which extended less than 2 vertebral bodies. Follow-up studies in MS patients revealed a reduction of both the spinal lesions and the cavities. It is still debated whether DS represents a distinct clinical entity different from MS. These findings help distinguishing both disorders in cases when spinal cavities are present and also contribute to the therapeutic choice.