OBJECTIVES: Hurricane Katrina's impact on public health has been significant and multifaceted, with trauma-related psychological sequelae likely to result in a sizable burden of disease. Data were collected that assessed acute stress disorder (ASD) prevalence and factors related to ASD symptomatology among sheltered evacuees. METHODS: On days 12 to 19 after Katrina, evacuees at a major emergency shelter completed surveys that assessed demographics, Katrina-specific experiences, and ASD symptomatology. RESULTS: Sixty-two percent of the sample met ASD threshold criterion. Projections based on the predictive power of ASD to posttraumatic stress disorder (PTSD) suggest that 38% to 49% of the sample will meet PTSD criteria 2 years post-disaster. Female gender (odds ratio [OR] = 4.08), positive psychiatric history (OR=5.84), injury (OR=2.75), increased life-threat perception (OR=1.37), and decreased sense of personal control (OR=1.56) were significantly related to ASD. Black race was associated with greater symptom severity (B=7.85, SE[B]=3.50). CONCLUSIONS: Katrina-related trauma and its psychological sequelae will remain a significant public health issue for years to come. The identification of several vulnerability factors related to ASD and PTSD provides a brief sketch of those at greatest risk.