Acquired polyneuropathies (PN) are rare in childhood and adolescent. We report on a 15-year-old male patient who presented with progressive gait instability, ataxia, neuropathic pain, distal muscle weakness and progressive loss of ambulation. Nerve conduction studies (NCS) revealed a progressive demyelinating sensorimotor polyneuropathy predominantly of the lower limbs. Cerebrospinal fluid (CSF) analyses revealed a cytoalbuminologic dissociation. Extensive diagnostic workup for autoantibodies and inflammatory markers was inconclusive. Corticosteroids and intravenous immunoglobulins did not affect. Cranial MRI revealed leptomeningeal enhancement of the cerebellum and the brainstem. Brain biopsy of the cerebellar lesions revealed an unclassifiable sarcoma. The patient was treated according to the CWS guidance study resulting in a decrease in enhanced lesion size. After two years NCS still revealed a demyelinating sensorimotor PN. This case report describes for the first time the clinical course of a chronic PN, putative paraneoplastic, associated with isolated unclassifiable CNS-sarcoma in an adolescent patient. Paraneoplastic pathogenesis should be considered in an unusual sequence of subacute progressive neurological symptoms even in children and adolescents.