Syringomyelia associated with posterior fossa tumours is a very infrequent combination of pathological entities. The few cases which have been reported generally were asymptomatic in respect of the spinal cavitations. The authors report on a 36-year-old woman with a large extradural posterior fossa epidermoid tumour with a concomitant holocord symptomatic syringomyelia. Some of her symptoms were clearly attributed to the intraspinal cavitation. The lesions were both diagnosed by magnetic resonance imaging (MR). The patient did well after surgery of the brain lesion, with an objective improvement in her neurological status and a complete resolution of the syrinx documented by the MR 7 months after tumour removal. Syringomyelia in this case could be explained by blockage of the cerebrospinal fluid (CSF) circulation at the foramen magnum which in turn resulted in cranio-spinal pressure dissociation. This led to an accumulation of extracellular fluid (ECF) in the central canal, starting cavitation. Consequently, the syrinx was slowly expanded by the long-standing "slosh" effect of the systolic pressure waves. However, also via a distortion mechanism within the posterior fossa a pathologically plugged obex could have contributed to syrinx formation by means of preventing drainage of fluid from the ventricular CSF system.