Background: Limited evidence suggests variation in mortality of older critically ill adults across Europe. We aimed to investigate regional differences in mortality among very old ICU patients. Methods: Multilevel analysis of two international prospective cohort studies. We included patients >= 80 yr old from 322 ICUs located in 16 European countries. The primary outcome was mortality within 30 days from admission to the ICU. Results are presented as n (%) with 95% confidence intervals and odds ratios (ORs). Results: Of 8457 patients, 2944 (36.9% [35.9-38.0%]) died within 30 days. Crude mortality rates varied widely between participating countries (from 10.1% [6.4-15.6%] to 45.1% [41.1-49.2%] in the ICU and from 21.3% [16.3-28.9%] to 55.3% [51.1-59.5%] within 30 days). After adjustment for confounding variables, the variation in 30-day mortality between countries was substantially smaller than between ICUs (median OR 1.14 vs 1.58). Healthcare expenditure per capita (OR=0.84 per $1000 [0.75-0.94]) and social health insurance framework (OR=1.43 [1.01-2.01]) were associated with ICU mortality, but the direction and magnitude of these relationships was uncertain in 30-day follow-up. Volume of admissions was associated with lower mortality both in the ICU (OR=0.81 per 1000 annual ICU admissions [0.71-0.94]) and in 30-day follow-up (OR=0.86 [0.76-0.97]). Conclusion: The apparent variation in short-term mortality rates of older adults hospitalised in ICUs across Europe can be largely attributed to differences in the clinical profile of patients admitted. The volume-outcome relationship identified in this population requires further investigation.