A case of brainstem encephalitis of undetermined etiology is reported in 66-year-old woman who had a sudden onset of illness with left abducens palsy, nystagmus and ataxia. The symptoms progressed to complete paralysis of eye movements, dysphagia and left hemiparesis with generalized hyperreflexia. Examination of CSF, CT scan and MRI of the brain were normal. The patient died 4 months after onset of disease. Neuropathologic study disclosed in the brainstem numerous perivascular and nodular inflammatory cell infiltrations composed predominantly of lymphocytes T and B. Most intensive inflammation concerned midbrain and pontine tegmentum and to a lesser degree medulla oblongata, pontine nuclei and cerebellar nuclei. Basal ganglia, cerebral and cerebellar cortex were unaffected. Neuropathological finding was reminiscent of brainstem encephalitides related to viral infection or to paraneoplastic syndrome. However, HSV-1, EBV, and CMV antigens were not detected by immunohistochemistry, as well as evidences of malignancy were not present in this case.