Rates and predictors of posttraumatic stress disorder (PTSD) over time are not well understood. This study is the first to look at the rates of PTSD immediately following war and 2 years later using a large cohort (N = 2,949) of Gulf War veterans. Using a cut score to indicate presumptive PTSD, 3% of participants exceeded the cutoff at Time 1 compared with 8% at Time 2. Those who exceeded the cutpoint at Time 1 were up to 20 times more likely to exceed the cutpoint at Time 2 than those who did not exceed the cutpoint at Time 1. Women and those with high levels of combat exposure were at increased risk for PTSD at both times. Being young, being single, and having previous combat experience were associated with increased risk at Time 1 only, whereas reservists and enlisted personnel were at increased risk at Time 2 only. These findings indicate that, although low initially, rates of PTSD increased substantially over time.