Within days after Hurricane Katrina in September 2005, the Houston-Harris County Immunization Registry was connected to the Louisiana Immunization Network for Kids Statewide. This linkage provided immediate access to the immunization records of children who were forced to evacuate the New Orleans, Louisiana, area. One year later, >18,900 immunization records have been found, representing an estimated cost savings of more than $1.6 million for vaccine alone and $3.04 million for vaccine plus administration fees. This experience demonstrated the vital and previously unrecognized functionality of immunization information systems in a public health emergency. Here we describe the Houston-Harris County Immunization Registry's experience after Hurricane Katrina in terms of maximizing the use of immunization information systems and the implications of this experience for patients, providers, and public health for future disaster-preparedness planning.