A case of gliomatosis cerebri in a 27-year-old man showing various cranial nerve manifestations is described. He was diagnosed as having cranial mononeuritis multiplex (bilateral oculomotor nerve paralysis, left facial nerve paralysis, bulbar palsy manifestations, and hypoglossal nerve paralysis) and was hospitalized in the neurology department on August 1, 2000. Although he continued to visit the neurology department after discharge, his manifestations showed no improvement. He was sent to our department for brain biopsy in August 2001. A biopsy performed at the Sylvian fissure from the frontal lobe/temporal lobe cortex showed high intensity on T2-weighted and Flair magnetic resonance imaging (MRI). The pathological findings were diffuse low-grade astrocytoma infiltrating between the pia mater and the cerebral cortex. We believed that the astrocytoma spreading on the subpia mater was responsible for the various cranial nerve manifestations, and we started whole-brain irradiation (46 Gy) + interferon (IFN)-β D.I.V. from September 2001. The pathological findings of the brain biopsy showed diffuse astrocytoma. The clinical presentation was dramatically improved after radiotherapy. It seemed that this tumor had spread along the subpia mater and subependyma. When he was discharged in early December, he walked by himself. The characteristic features of this case are that no lesion in the cerebellum or brain stem was found on MRI, even though the main manifestations were cerebello-brain stem manifestations, and biopsy of the cerebral cortex revealed astrocytoma. It should be noted that the clinical manifestations of astrocytoma in some cases are dissociated from the imaging observations.