 The purpose of this research is to evaluate the usefulness of memantine, compared to placebo sugar pill, for the treatment of cognitive impairment in patients with idiopathic Parkinsons disease PD and dementia. Memantine is used as a safe and effective treatment for patients with Alzheimers disease. Cognitive impairment includes concentration and memory difficulties. We will look at how well this medication helps your cognitive impairment, how well you tolerate this medication including its effects on your motor symptoms of PD your activities of daily living, your emotions, and any medical conditions you might have. We will interview a person you choose as your informant. 