Brainstem gliomas are rare primary brain tumours which most commonly occur in the midbrain and pons. Malignant gliomas and tumours at the cervico-medullary junction are particularly unusual. The diagnosis of tumours at this site is particularly difficult using computed tomographic (CT) scanning owing to artifacts around the base of the skull. Intrinsic tumours of the cervico-medullary junction may lead to a dissociated motor deficit and the onset of symptoms can be rapid. We describe a patient in whom an isolated ascending motor deficit in association with a raised cerebrospinal fluid protein and a normal CT scan led to an erroneous diagnosis of Guillain-Barré syndrome. The patient was treated on the intensive care unit for an 8-week period before further investigation demonstrated a malignant glioma of the cervico-medullary junction. We recommend confirmation of the diagnosis of polyradiculopathy by nerve conduction studies wherever possible.