Data from the National Vietnam Veterans Readjustment Study, conducted from 1986 to 1988, were used to develop and cross-validate a model of the etiology of posttraumatic stress disorder (PTSD) among a community sample of 1198 male Vietnam theater veterans. The initial model specified causal paths among five sets of variables, ordered according to their historical occurrence: a) premilitary risk factors and traumas, b) war-related and non-war-related traumas during the military, c) homecoming reception, d) postmilitary traumas, and e) PTSD. The initial model was refined and then cross-validated, leading to the specification of a final model with highly satisfactory fit and parsimony. In terms of the magnitude of their contribution to the development of PTSD, lack of support from family and friends at the time of the homecoming and exposure to combat were the two most influential contributors. Other contributing factors, in order of importance, were Hispanic ethnicity, societal rejection at the time of homecoming, childhood abuse, participation in abusive violence, and family instability. Exposure to war-related and non-war-related traumas occurred largely independently of each other, with war-related traumas contributing substantially more than non-war-related traumas to the development of PTSD. Limitations to interpretation of the results are noted due to the retrospective nature of the data and the inevitable omission of other etiological factors.