A significant proportion of brain tumor patients treated with radiation or chemotherapy who are in disease remission experience cognitive sequelae from their treatment. Cognitive dysfunction can be of sufficient severity to interfere with their ability to function at premorbid professional and social levels. There are, however, no approved pharmacological interventions for improving cognitive functions in cancer patients who display treatment-related cognitive deficits. Donepezil, an acetylcholinesterase inhibitor, has been shown to provide cognitive and functional benefits in patients with Alzheimer's disease, vascular dementia, and in patients with other neurological diseases without known cholinergic deficiency. The proposed pilot study will examine the efficacy of donepezil in improving cognitive functions in adult brain tumor patients treated with radiation and/or chemotherapy who have mild to moderate cognitive difficulties. Neuropsychological measures of executive, psychomotor speed, attention, and memory abilities will be administered prior to, during and following donepezil therapy. The proposed study will also test the hypothesis that the apolipoprotein E (APOE) e-4 allele correlates with the development of cognitive impairment after radiation or chemotherapy treatments. The proposed investigation is unprecedented and may provide preliminary information about (1) a pharmacological therapy that could improve cognitive functions in this population, and (2) a genetic risk factor that may increase vulnerability to radiation or chemotherapy-induced cognitive decline.