Background: Spinal dermoid cysts are uncommon, benign tumours of ectodermal origin, often associated with spinal dysraphism. Malignant transformation of spinal dermoid cysts is an exceptionally rare entity, with transformation into carcinosarcoma not previously reported. Methods: Case report and literature review Results: A 41-year-old male presented with a recurrent lumbar intradural mass, 28 years after resection of a dermal sinus tract and associated dermoid cyst. Intraoperative appearance and subsequent pathology were again consistent with a dermoid cyst. The patient re-presented 2 weeks after surgery with diplopia and headache due to hydrocephalus, thought to be due to chemical meningitis. Following ventriculoperitoneal shunt implantation, the patient rapidly deteriorated with progressive neurologic deficits and widespread leptomeningeal enhancement. A repeat spinal leptomeningeal biopsy was pursued, which revealed malignant transformation of the dermoid cyst into invasive carcinosarcoma. Without curative treatment options, the patient was palliated and died 85 days after admission. Conclusions: Malignant transformation of spinal dermoid cysts should be considered in the differential diagnosis of patients with dermoid cysts and progressive leptomeningeal enhancement. False negatives can occur with initial tumour pathology and repeat sampling may be warranted for diagnostic clarity. To the authors knowledge, this is the first report of a spinal dermoid cyst with malignant transformation into carcinosarcoma.