Background: Delirium has not been found to be a significant predictor of postdischarge mortality, but previous research has methodologic limitations including small sample sizes and inadequate control of confounding. This study aimed to determine the independent effects of presence of delirium, type of delirium (incident vs prevalent), and severity of delirium symptoms on 12-month mortality among older medical inpatients. Methods: A prospective, observational study of 2 cohorts of medical inpatients was conducted with patients 65 years or older: 243 patients had prevalent or incident delirium, and 118 controls had no delirium. Baseline measures included presence of delirium and/or dementia, severity of delirium symptoms, physical function, comorbidity, and physiological and clinical severity of illness. Mortality during the 12 months after enrollment was analyzed with the Cox proportional hazards model with adjustment for covariates. Results: The unadjusted hazard ratio of delirium with mortality was 3.44 (95% confidence interval, 2.05-5.75); the adjusted hazard ratio was 2.11 (95% confidence interval, 1.18-3.77). The effect of delirium was sustained over the entire 12-month period after adjustment for covariates and was stronger among patients without dementia. Among patients with dementia, there was a weak, nonsignificant effect of delirium on survival. After adjustment for covariates, mortality did not differ between patients with incident and prevalent delirium, but among patients with delirium without dementia, greater severity of delirium symptoms was associated with higher mortality. Conclusions: Delirium is an independent marker for increased mortality among older medical inpatients during the 12 months after hospital admission. It is a particularly important prognostic marker among patients without dementia. [ABSTRACT FROM AUTHOR]