Summary:The Dementia Severity Rating Scale (DSRS) is an informant-based, multiple- choice questionnaire that assesses severity from the mildest to the most severe stages in the major functional and cognitive domains affected in Alzheimer disease (AD). The DSRS has good reliability as measured by making repeated observations over short time periods and by comparing caregiver responses to information collected by a physician or an experienced nonphysician research associate. The measure correlates favorably with the Washington University Clinical Dementia Rating scale and with standard cognitive testing measures. The DSRS provides a simple, valid, and sensitive measure of impairment associated with AD. It is ideally suited for multisite collaborative studies and can be implemented with a minimum of staff time and training.