The purpose of this review was to summarize research on the clinical benefits of early treatment for Alzheimer's disease via a focused discussion of data on donepezil. Well-controlled clinical trials demonstrate that donepezil is effective in stabilizing or slowing progressive decline in cognition, function, and behavior. Several studies reveal statistically significant and clinically meaningful advantages to initiating treatment early in the course of the disease. Benefits of donepezil treatment include behavioral stabilization and preserved independence, in addition to slowed cognitive decline. Epidemiologic studies and evidence from histopathology underlie a rationale for treating patients who are cognitively impaired but do not have dementia. Clinical trials in such patients indicate that treating patients with mild cognitive impairment (MCI) may delay the onset of Alzheimer's dementia. Substantial evidence favors initiating treatment early in the course of dementia and reinforces the necessity to assess behavior and activities of daily living to accurately evaluate treatment response. Results of early studies of donepezil in MCI are promising and suggest directions for further research.