A 63-year-old woman showed the dominant clinical features of aphasia as the initial symptom in the course of presenile progressive dementia. Computerized axial tomography showed strikingly dilated Sylvian fissures, particularly in its posterior aspect, in the presence of moderate diffuse cortical atrophy. EEG revealed a slowdown in activity with the dominance in the left fronto-temporal area. The radiographic and EEG findings correlated well with the clinical data. This case was compared with those of presenile dementia previously reported. This is a unique case report of presenile dementia.