Objective: To demonstrate the use of cost-effectiveness analysis to assess the economic impact of donepezil in the treatment of mild or moderate AD. Background: Cost-effectiveness analyses show the relationship between resources used (costs) and health benefits achieved (effects) for an intervention compared with an alternative strategy. Methods: We developed a model to estimate the incremental cost-effectiveness of donepezil compared with no treatment. We determined costs per quality-adjusted life-years gained, a measurement that enhances the comparability of diverse studies. The model projects the progression of AD patients into more severe disease stages and into nursing homes. Data from a randomized clinical trial of donepezil were used to assess the drug's impact on the 6-week probabilities of progression. Data on the costs and health-related quality of life associated with different disease stages and settings were taken from published estimates and our companion cross-sectional study, respectively. Results: Donepezil costs are partially offset by a reduction in the costs of care due to enhancement in cognitive functioning and the delay to more costly disease stages and settings. The magnitude of this cost offset and of the effect of donepezil on health-related quality of life depends on the model's assumptions about the duration of the drug effect, where controlled data are lacking. If the drug effect exceeds 2 years, the model predicts that for mild AD the drug would pay for itself in terms of cost offsets. Conclusions: The results of the cost-effectiveness model presented here suggest that donepezil may be cost-effective but additional controlled data on long-term drug efficacy are needed.