Dysplastic structural lesions within the fourth ventricle have been reported in patients with Chiari malformation type II (CM-II). The authors report the unique case of a 22-year-old patient with myelomeningocele who presented with progressive pain, decreasing hand function, and lower cranial nerve dysfunction in the context of a slowly enlarging intramedullary mass at the cervicomedullary junction. At surgery a multinodular mass attached to caudally displaced fourth ventricle choroid plexus was completely removed from an expanded central canal. The histopathological findings were consistent with dysplastic-reactive choroid plexus. Postoperatively the patient experienced relief of pain and improvement in hand strength. To the authors' knowledge, this is the first reported case in which dysplastic fourth ventricle choroid plexus was displaced caudally through the obex to become an intramedullary lesion at the cervicomedullary junction. Its subsequent slow enlargement with progressive neurological deficits may have been secondary to reactive inflammatory changes. For patients with myelomeningocele and CM-II, intramedullary dysplastic-reactive choroid plexus should be included in the differential diagnosis of mass lesions in this location.