The cognitive subscale of the Alzheimer's Disease Assessment Scale (ADAS-Cog) is designed specifically to assess memory, language, and praxis dysfunctions characteristic of Alzheimer's disease (AD). In this report, we use data from 1,648 AD participants in two identical 26-week multicenter drug trials to derive clinical trial population-based norms for ADAS-Cog subtest scores. All 11 ADAS-Cog item scores were sensitive to differences in dementia severity judged either by baseline Mini-Mental State Exam (MMSE) scores or by Global Deterioration Scale (GDS) stage of dementia. Using ADAS-Cog subtest scores alone, 85 percent of GDS 4 subjects and 69 percent of GDS 5 subjects could be classified correctly. In a stepwise discriminant analysis, orientation was the item with the greatest discriminatory power between subjects with GDS 4 and GDS 5 stages of dementia. ADAS-Cog subtest scores also varied by age, gender, educational level, and concurrent use of anti-inflammatory drugs or estrogen (in women). Such normative data may facilitate the selection of appropriate outcome measures to investigate differential treatment effects on specific cognitive domains or in specific AD subpopulations.