A 23-year-old man presented with a 2-week history of intracranial hypertension. CT showed a large, nonenhancing cerebellar mass with surrounding calcification and displacement of the fourth ventricle. MRI revealed a septate lesion, with low signal on T1-weighted and high signal on T2-weighted images. The cerebellar tonsils were displaced below the foramen magnum and there was associated syringomyelia. The MRI features were characteristic of Lhermitte-Duclos (LD) disease (dysplastic gangliocytoma) and the diagnosis was confirmed following surgery. In this case, we emphasise the usefulness of MRI in the diagnosis of LD disease and consider the possible pathogenesis of the associated syringomyelia.