Sixty-four cases of idiopathic Parkinson's disease were studied focusing the neuropsychological performance. To all subjects a brief mental status questionnaire was administered (30 items). Cases with a performance below normal and meeting the dementia criteria of the Diagnosis and Statistical Manual of Mental Disorders, third edition (DSM III), were considered demented. This evaluation showed that 12 of the 64 patients had dementia, a prevalence rate of 18.7%. Patients that had neuropsychological performance below normal showed more severe bradykinesia than normals. Short term memory, abstract thinking, visual gnosia, calculations, dynamic motor function of hands, constructional ability and recent memory were the most affected psychological functions. The significance of the mentioned findings is discussed and the literature reviewed.