The threat of mass-casualty disasters raises concern about the adequacy of hospitals’ surge capacity. This paper measures surge capacity as the number of empty staffed beds per capita at the county level and compares it to a disaster-planning benchmark released by the federal government. The percentage of the U.S. population living in counties falling below the benchmark increased from 19 percent in 2000 to 30 percent in 2005. Limitations in surge capacity are associated with rapid population growth but not with higher rates of hospital use. Limitations appear to be most severe along the East Coast and in the West.