To better characterize response to donepezil in patients with severe AD, Severe Impairment Battery (SIB) data were pooled from four donepezil clinical trials (N=904). Changes in SIB total and domain scores from baseline to week 24 were compared between placebo and donepezil treatment groups (observed case analysis). Analyses were stratified by baseline severity (Mini-Mental State Examination [MMSE] scores 1-5, 6-9, 10-12 and 13-17) to allow investigation of responses at different stages of cognitive impairment. Relationships to global and functional measures were explored. The difference between donepezil- and placebo-treated patients in least squares (LS) mean change in SIB total scores from baseline to week 24 was 6.22 (p <0.0001, Cohen's d, 0.53). Treatment-placebo differences were statistically significant for each baseline severity stratum, being greatest for the MMSE 6-9 stratum (LS mean difference, 7.60; p <0.0001, Cohen's d, 0.66). Treatment-placebo differences in LS mean change in SIB domain scores significantly favored donepezil for seven of nine domains (range, p = 0.0056 to p <0.0001; Cohen's d, 0.17-0.48). Change in total SIB score correlated significantly with change in measures of activities of daily living and global status. These results indicate that donepezil provides cognitive benefits in patients with severe AD, including those most markedly impaired. The treatment effect size and correlation between improvements in SIB scores and functional and global outcome measures suggest the drug-placebo differences are clinically meaningful.