The authors evaluated the validity, reliability, and sensitivity to change of the Delirium Severity Scale (DSS), a 10-minute assessment consisting of Forward Digit Span and Similarities. Twenty-two older inpatients with delirium but not dementia and 15 control patients were administered the DSS during hospitalization. Scores were significantly inversely correlated with experts' quantitative ratings of severity at all three time-points examined. The DSS sbowed significant improvement over time (P <0.001) and significant correlation with improvement in expert ratings (P = 0.026). The DSS shows promise as a valid and reliable measure sensitive to changing symptom severity.