Context: Delirium is a frequent occurrence in older ICU patients, but the importance of the duration of delirium in contributing to adverse long-term outcomes is unclear. Objective: Examine the association of the number of days of ICU delirium with mortality in an older patient population. Design: Prospective cohort study. Setting: Fourteen-bed intensive care unit in an urban acute care hospital. Patients: 304 consecutive admissions age 60 years and older. Main Study Measures: The main outcome was one-year mortality after ICU admission. Patients were assessed daily for delirium with the Confusion Assessment Method for the ICU (CAM-ICU) and a validated chart review method. Results: The median duration of ICU delirium was 3 days (range 1-46). During the course of follow-up, 153 (50%) patients died. After adjusting for relevant covariates, including age, severity of illness, comorbid conditions, psychoactive medication use, baseline cognitive and functional status, the number of days of ICU delirium was significantly associated with time to death within one year post-ICU admission (HR, 1.10; 95% CI, 1.02-1.18). Conclusion: Number of days of ICU delirium was associated with higher 1-year mortality after adjustment for relevant covariates in an older ICU population. Investigations should be undertaken to reduce the number of days of ICU delirium and to study the impact of this reduction on important health outcomes including mortality and functional and cognitive status.