Hurricane Katrina was the most devastating natural disaster to hit the United States in the past 75 years. The authors conducted interviews of 810 persons who were representative of adult residents living in the 23 southernmost counties of Mississippi before Hurricane Katrina. The prevalence of posttraumatic stress disorder (PTSD) since Hurricane Katrina was 22.5%. The determinants of PTSD were female gender, experience of hurricane-related financial loss, postdisaster stressors, low social support, and postdisaster traumatic events. Kaplan-Meier survival curves suggest that exposure to both hurricane-related traumatic events and to financial and social stressors influenced the duration of PTSD symptoms. Postdisaster interventions that aim to improve manipulable stressors after these events may influence the onset and course of PTSD.