BACKGROUND: The cognitive subscale of the Alzheimer's Disease Assessment Scale(ADAS-Cog) has been extensively valiated in assessing cognitive function in patients with Alzheimer's Disease(AD) and used as an efficacy measure in clinical trials of AD. There is a need for additional data on the relationship between cognitive performance and other measures of dementia to fully assess the value of the ADAS-Cog as a measure of treatment efficacy. METHODS: We used data from 53 AD participants in 8 multicenter clinical drug trials to examine the distribution of baseline ADAS-Cog scores in relation to Mini-Mental State Examination(MMSE), Global Deterioration Scale(GDES), Katz Index of Activity of Daily Living(ADL), Lawton Instrumental Activity of Daily Living(IADL), Geriatric Depression Scale(GDS) and Mini-Nutritional Assessment(MNA). RESULTS: The ADAS-Cog score was statistically significantly correlated with MMSE (R=-0.755, P<0.001), GDES(R=0.403, P<0.003), ADL(R=-0.532, P<0.001), IADL(R=-0.626, P<0.001) and MNA(R=-0.427, P=0.004) scores. But GDS scores were not associated with ADAS-Cog scores.(R= -0.123, P=0.396) CONCLUSIONS: This study characterizes the relationship between ADAS-Cog scores and other commonly used measures of dementia in AD patients. As expected, baseline scores on ADAS-Cog and MMSE demonstrated significantly high correlation. The relatively weaker correlation between ADAS-Cog and GDES scores may be related to the fact that GDES does not evaluate cognitive function only. There are also significant correlations between ADAS-Cog and ADL, IADL, and MNA which indicate the severity and progression of dementia in AD patients. Further studies with larger samples including cognitive function of broader spectrum need to confirm the findings in this study.