Hematologists/oncologists would provide essential care for victims of a catastrophic radiation incident, such as the detonation of an improvised nuclear device (IND). The US Radiation Injury Treatment Network (RITN) is a voluntary consortium of 37 academic medical centers, 8 blood donor centers, and 7 umbilical cord banks focused on preparedness for radiation incidents. The RITN conducted 2 tabletop exercises to evaluate response capability after a hypothetical IND detonation in a U.S. city. In the 2008 exercise, medical centers voluntarily accepted 1757 victims at their institutions, a small fraction of the number in need. In the 2009 exercise, each center was required to accept 300 victims. In response, the centers outlined multiple strategies to increase bed availability, extend staff and resources, and support family and friends accompanying transferred victims. The exercises highlighted shortcomings in current planning and future steps for improving surge capacity that are applicable to various mass casualty scenarios.