As many as four million Americans were stationed in Indo-china during the Vietnam War; some 800,000 were assigned to the combat zone. These individuals are at especially high risk for developing posttraumatic stress disorder, a diagnostic category that appeared for the first time in DSM-III. The authors explore the reasons why many Vietnam veterans have developed the disorder. They discuss four broad types of disorders now being found among veterans, the problems in diagnosing and treating the disorder, and the theoretical basis in ego psychology for the behavior of veterans with the disorder. Also briefly outlined are some clinical considerations for therapists treating Vietnam veterans.