The present paper is a study of a microcystic intraspinal tumour that developed in a 4-year-old child. The histologic examination showed the presence of neuroblastic elements developing next to smooth muscle fibers. The tumour had diffusely invaded the meninges, presenting a marked collagen reaction. Ultrastructurally, the neuroblastic elements exhibited an arrangement proper to the neural tube and the presence of a series of osmiophil neurosecretory dense-core vesicles. The fine structural morphology of the smooth muscle fibers was clearly revealed, the cells containing microfibrils and being delimited by a basement membrane. The muscular fibers were visibly in contact with the blood vessels. These aspects demonstrate that the medullomyoblastoma is a malignant bidermal teratoid tumour of the central nervous system. Dysgenesis of the ectomesenchyma may be considered to stand at the basis of the development of this type of tumour.