Background: Very elderly critically ill patients (ie, those older than 75 or 80 years) are an increasing population in intensive care units. However, patients with cancer have encompassed only a minority in epidemiological studies of very old critically-ill patients. We aimed to describe clinical characteristics and identify factors associated with hospital mortality in a cohort of patients aged 80 or older with cancer admitted to intensive care units (ICUs). Methods: This was a retrospective cohort study in 94 ICUs in Brazil. We included patients aged 80 years or older with active cancer who had an unplanned admission. We performed a mixed effect logistic regression model to identify variables independently associated with hospital mortality. Results: Of 4604 included patients, 1807 (39.2%) died in hospital. Solid metastatic (OR = 2.46; CI 95%, 2.01-3.00), hematological cancer (OR = 2.32; CI 95%, 1.75-3.09), moderate/severe performance status impairment (OR = 1.59; CI 95%, 1.33-1.90) and use of vasopressors (OR = 4.74; CI 95%, 3.88-5.79), mechanical ventilation (OR = 1.54; CI 95%, 1.25-1.89) and renal replacement (OR = 1.81; CI 95%, 1.29-2.55) therapy were independently associated with increased hospital mortality. Emergency surgical admissions were associated with lower mortality compared to medical admissions (OR = 0.71; CI 95%, 0.52-0.96). Conclusions: Hospital mortality rate in very elderly critically ill patients with cancer with unplanned ICU admissions are lower than expected a priori. Cancer characteristics, performance status impairment and acute organ dysfunctions are associated with increased mortality.