Other articles in this series can be found online at http://ccforum.com/articles/ theme-series.asp?series=CC_Disaster Clinical review: Mass casualty triage – pandemic influenza and critical care Worst case scenarios for pandemic influenza planning in the US involve over 700,000 patients requiring mechanical ventilation. UK planning predicts a 231% occupancy of current level 3 (intensive care unit) bed capacity. Critical care planners need to recognise that mortality is likely to be high and the risk to healthcare workers significant. Contingency planning should, therefore, be multi-faceted, involving a robust health command structure, the facility to expand critical care provision in terms of space, equipment and staff and cohorting of affected patients in the early stages. It should also be recognised that despite this expansion of critical care, demand will exceed supply and a process for triage needs to be developed that is valid, reproducible, transparent and consistent with distributive justice. We advocate the development and validation of physiological scores for use as a triage tool, coupled with candid public discussion of the process. It is widely accepted that conditions exist for the evolution of a new strain of influenza virus with the potential to cause a human pandemic [1] . The biggest challenge in planning for an influenza pandemic is the range of unknown factors; its nature and impact cannot be fully predicted until the pandemic virus actually emerges. Those planning for a pandemic must, therefore, work from a number of assumptions based on knowledge gained from previous pandemics and scientific modelling of a range of potential scenarios. The UK Pandemic Influenza Plan [2] sets out a range of possible scenarios for clinical attack rates and case fatality rates during a pandemic, including the potential for more than one wave. The base scenario assumes a clinical attack rate of 25% and a case fatality rate of 0.37%, giving rise to 53,700 excess deaths in the UK. A reasonable worst case scenario involves a cumulative clinical attack rate of 50% with 2.5% case fatality, causing 709,300 excess deaths. Similarly, the US Department of Health and Human Services predicts that in a "moderate" scenario based on a virus with 1968-like pathogenicity, 865,000 will require hospitalisation and 65,000 (7.5%) will require ventilation. They also outline a "severe" 1918-like scenario with 9.9 million hospitalisations and 743,000 patients requiring ventilation [3] .