Background:; Delirium is the most frequent complication associated with hospitalization of older adults, responsible for 17.5 million additional hospital days in the United States each year; yet, nurses fail to recognize it more than 30% of the time.; Objectives:; The specific aim of the study was to measure staff nurses' recognition of delirium in hospitalized older adults by comparing nurse and expert diagnostician ratings for delirium using the Confusion Assessment Method (CAM).; Method:; This study investigated the rate of agreement/disagreement between researchers and a convenience sample of 167 nurses caring for 170 medical surgical patients (965 years) in detecting delirium. Paired (nurse vs researcher) CAM ratings were completed at least every other day until either discharge or delirium was detected by the researcher.; Results:; The researcher detected delirium in 7% (12/170) of patients. Nurses failed to recognize delirium 75% (9/12) of the time, with poor agreement between nurse/researcher for all observations (kappa = 0.34). A generalized estimating equation logistic regression model identified independent predictors of nurses' underrecognition of delirium that included increasing age and length of stay, dementia, and hypoactive delirium.; Discussion:; Findings provide further support for the significance of nurses' underrecognition of delirium in the hospitalized older adult when using the CAM. Additional research is warranted regarding the clinical decision-making processes that nurses use in assessing acute cognitive changes and in identifying strategies to improve delirium recognition.