A 69-year-old male presented with a 4-month episode of tonic seizures of the right arm and leg. Neurological examination revealed no abnormal findings. In the electroencephalogram, sharp wave trains were seen dominantly in the left frontal region. Computed tomography scans showed a spotty hyperdense lesion in the left insular area with minimal enhancement. Left carotid angiogram demonstrated no abnormalities in the arterial and capillary phases. In the venous phase, however, a varicose dilatation of the posterior insular vein was found, which was also noticed in the delayed phase. Magnetic resonance T1-weighted images failed to detect the lesion, but T2-weighted images showed a slight signal focus in the same area. After administration of 300 mg of phenytoin daily, the frequency of attacks markedly decreased and the patient was kept under close outpatient observation.