We report a case of orbital plexiform neurofibroma presenting in a 10-year-old boy with von Recklinghausen's neurofibromatosis. The patient had shown a slow enlargement of exophthalmos of the right eye present since birth, together with multiple café au lait spots on the skin of the trunk. Magnetic resonance (MR) images revealed diffuse and irregular nodular involvement of the retrobulbar nerves within the muscle cone, which was confirmed at the surgery. The tumour extended into the ipsilateral cavernous sinus. We discuss the MR findings as pathognomonic signs of this rare orbital tumour, including its multinodular nature among dispersed intraconal fat tissue, location around the optic nerve, extension through the superior orbital fissure into the cavernous sinus and association with von Recklinghausen disease.