Background: Few studies have longitudinally examined predictors of posttraumatic stress disorder (PTSD) in a nationally representative sample of US veterans. We examined predictors of warzone-related PTSD over a 25-year span using data from the National Vietnam Veterans Longitudinal Study (NVVLS). Methods: The NVVLS is a follow-up study of Vietnam theater veterans (N = 699) previously assessed in the National Vietnam Veterans Readjustment Study (NVVRS), a large national-probability study conducted in the late 1980s. We examined the ability of 22 premilitary, warzone, and postmilitary variables to predict current warzone-related PTSD symptom severity and PTSD symptom change in male theater veterans participating in the NVVLS. Data included a self-report Health Questionnaire survey and a computer-assisted telephone Health Interview Survey. Primary outcomes were self-reported PTSD symptoms assessed by the PTSD Checklist for DSM-5 (PCL 5) and Mississippi PTSD Scale (M-PTSD). Results: Predictors of current PTSD symptoms most robust in hierarchical multivariable models were African-American race, lower education level, negative homecoming reception, lower current social support, and greater past-year stress. PTSD symptoms remained largely stable over time, and symptom exacerbation was predicted by African-American race, lower education level, younger age at entry into Vietnam, greater combat exposure, lower current social support, and greater past-year stressors. Conclusions: Findings confirm the robustness of a select set of risk factors for warzone-related PTSD, establishing that these factors can predict PTSD symptom severity and symptom change up to 40 years postdeployment.