The aim of the study was to assess the reliability of magnetic resonance imaging (MRI) in the detection of syrinx cavities and to determine the proportion of cavities associated with Chiari malformation. MRI was performed in over 120 patients with neurologic deficits referable to the cranio-cervical junction. Forty-one patients were also evaluated by CT myelography (CTM). CTM demonstrated appearances consistent with intramedullary cavitation in 14 patients. MRI detected 11 of these but in 3 the appearances, including cord size, were normal. In the entire study 27 syrinx cavities were demonstrated by MRI, 8 associated with trauma, posterior fossa or spinal tumours, or spinal dysraphism. Of the remaining 19, tonsillar ectopia was present in 10 (53%). It is concluded that a lack of concordance between MRI and CTM exists in the detection of small cavities in the normal sized cord, and that syringomyelia without evidence of tonsillar ectopia is identifiable by MRI more frequently than might be anticipated.