Donepezil is a highly selective, long-acting acetylcholinesterase inhibitor which has been approved in numerous countries for the treatment of Alzheimer's disease. Donepezil increases the levels of acetylcholine in the brain of patients with Alzheimer's disease and presumably corrects some dysfunction associated with loss of cells in the basal forebrain. Several large-scale multinational clinical studies have demonstrated drug-related improvements in cognitive performance on objective psychometric tests and in ratings by clinicians of patients' function. Further studies will be needed to determine the long-term impact of donepezil on Alzheimer's disease and related dementias.