Hospitals and healthcare workers face the challenge of being prepared to manage victims of acts of terrorism that involve chemical, biological, and radiological agents that they do not commonly encounter. One example that is often cited as a potential terrorism scenario is the use of a conventional explosive that is mixed with radioactive material. On November 10, 2004, we conducted a regional multihospital full-scale exercise involving 11 hospitals and 358 victim-observers to evaluate hospital preparedness for such an event. Our results demonstrate that hospitals are not adequately prepared to manage mass casualties with associated radiological contamination.