Study objective We seek to determine how delirium and dementia affect the accuracy of the presenting illness and discharge instruction comprehension in older emergency department (ED) patients. Methods This cross-sectional study was conducted at an academic ED from May 2008 to July 2008 and included non-nursing home patients aged 65 years and older. Two open-ended interviews were performed to assess patients' ability to accurately provide their presenting illness and comprehension of their ED discharge instructions. The surrogates' version of the presenting illness and printed discharge instructions were the reference standards. Concordance between the patient and the reference standards was determined by 2 reviewers using a 5-point scale ranging from 1 (no concordance) to 5 (complete concordance). Proportional odds logistic regression was performed to determine whether cognitive impairment was associated with presenting complaint accuracy and discharge instruction comprehension. All models were adjusted for age, health literacy, education, nonwhite race, and hearing impairment. Results For the presenting illness analysis, 202 patients participated. Compared with patients without cognitive impairment, those with delirium superimposed on dementia (DSD) had lower odds of agreeing with their surrogates with regard to why they were in the ED (adjusted proportional odds ratio=0.20; 95% confidence interval CI 0.09 to 0.43). For the discharge instruction comprehension analysis, 115 patients participated. Patients with DSD had significantly lower odds of comprehending their discharge diagnosis (adjusted proportional odds ratio=0.13; 95% CI 0.04 to 0.47), return to the ED instructions (adjusted proportional odds ratio=0.18; 95% CI 0.04 to 0.82), and follow-up instructions (adjusted proportional odds ratio=0.09; 95% CI 0.02 to 0.35) compared with patients without cognitive impairment. Conclusion DSD is associated with decreased accuracy of the older patient's presenting illness and decreased comprehension of ED discharge instructions.