Mood and behavioral changes have been reported in patients with thalamic tumors. We report a case in which an infiltrating mass lesion of both thalami was manifested by an alteration in personality with relative motor and sensory sparing. These personality changes, unlike many such cases reported in the literature, were not related to increased intracranial pressure and hydrocephalus. Magnetic resonance imaging (MRI) demonstrated the lesion that was subtle and nearly inapparent on contrast-enhanced computerized tomography (CT).