During the week after Hurricane Katrina struck the Gulf Coast on August 29, 2005, an estimated 240,000 persons, mostly from Louisiana, evacuated to Houston, Texas. On August 31, an estimated 24,000 evacuees were sheltered temporarily at facilities in Reliant Park, a sports and convention complex that includes Reliant Astrodome, Reliant Center, and Reliant Arena. All evacuees to these three facilities were provided with cots, bedding, food, water, and access to lavatories and showers. A medical facility was set up initially to provide emergency care to evacuees and subsequently to serve as a comprehensive outpatient clinic staffed largely by personnel from the Harris County Hospital District (HCHD), Baylor College of Medicine (BCM), and Texas Children's Hospital (TCH). On September 2, 2005, physicians and staff from Harris County Public Health and Environmental Services (HCPHES) noted a substantial number of adults and children with symptoms of acute gastroenteritis (defined as diarrhea and/or vomiting) at the medical clinic in Reliant Park. In collaboration with HCPHES, CDC and medical personnel of HCHD, BCM, and TCH conducted enhanced surveillance to improve identification of acute gastroenteritis, investigate the apparent outbreak, identify the infectious agent, and implement measures for its control. This report summarizes the preliminary epidemiologic data from this investigation and underscores the challenges to managing a large and rapidly spreading outbreak of norovirus in crowded evacuee settings.