Background: Unplanned admission to the intensive care unit has been shown to significantly increase the risk of inhospital mortality. Medical advances and increased expectations have resulted in a greater number of very elderly patients (80 years and over) being admitted to the ICU. The risk of in-hospital death associated with unplanned admission to the ICU in very elderly patients has not been clearly defined. Objective: To estimate the risk of in-hospital mortality associated with unplanned admission to the ICU in patients aged 80 years and over. Design, setting and participants: Retrospective review of an adult intensive care database. The setting was Liverpool Hospital, a large teaching hospital in Sydney, Australia, with a 28-bed ICU that has about 2000 admissions per year. We analysed data on very elderly patients (n = 1680), aged 80 years or more, admitted to the ICU between 1 January 1997 and 31 December 2007. Main outcome measures: Baseline risk factors for inhospital mortality. Results: Mortality among patients with unplanned ICU admissions was 47%, compared with 25% in patients with planned admissions (adjusted rate ratio [RR], 1.92 [95% CI, 1.59���2.32]). An estimated 50% of the overall risk of inhospital death among very elderly patients was attributable to a combination of unplanned admission to the ICU, the presence of at least one comorbid condition, acute renal failure and respiratory failure requiring intubation. Conclusion: Unplanned admission to the ICU increases the risk of in-hospital mortality in very elderly patients. At least 50% of the risk of in-hospital death in this age group is attributable to a combination of unplanned ICU admission, comorbidity (���1 comorbid condition), acute renal failure and respiratory failure.