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BACKGROUND: A cerebral spinal fluid (CSF) cavity within the conus medullaris has been described by the term ventriculus terminalis (VT) or the fifth ventricle. The finding of a VT on MRI imaging of the lumbar spine is often incidental but may be found in patients with low back pain or neuromuscular deficits. These lesions, when identified, are thought to regress or remain stable in terms of size, although some have been described to enlarge in the presence of post-traumatic meningeal hemorrhages or deformities of the vertebral canal. CASE REPORT: We describe a case of a slowly growing VT in a patient with progressing lower limb weakness without any history or imaging findings of trauma or spinal canal abnormalities. CONCLUSIONS: We present an intriguing case of a slowly growing VT in a woman with progressive neurological symptoms. Surgical fenestration provided complete symptomatic relief and follow-up imaging two years after surgery demonstrated no evidence of recurrence. This, to our knowledge, is the first described case of a slowly enlarging VT independent of any other imaging findings.