The sensitivity of reaction time (RT) measurement in assessing cognitive decline in dementia of the Alzheimer type (DAT) was evaluated. Sixteen DAT patients and 16 normal elderly controls, matched for age and education, read 50 stimulus words presented individually on a cathode-ray tube. DAT patients exhibited a significantly greater cognitive impairment as shown by multivariate analysis of variance (p less than 0.0001). Canonical variate correlations revealed RT (0.961) to be more sensitive to decline in cognitive functioning in dementia than the measure of number of reading errors (0.559). Results confirm that RT is a highly sensitive measure of central nervous system integrity, which allows for a more refined investigation of a patient's cognitive deficit than is obtainable with traditional test measures. These findings suggest that chronometric measures should be incorporated in clinical and research studies attempting to document the existence and severity of cognitive impairment in neurodegenerative disease.