To estimate the relative contributions of trauma and premorbid disposition in the development and persistence of posttraumatic stress disorder (PTSD) symptoms, we conducted structured psychiatric interviews of 62 former World War II POWs. Half these men satisfied DSM-III criteria for PTSD in the year following repatriation. Eighteen (29%) continued to meet the criteria for PTSD 40 years later. Family history of mental illness and preexisting psychopathology were at best only weakly correlated with persistent PTSD symptoms. The strongest predictors of PTSD were proportion of body weight lost and the experience of torture during captivity. This study demonstrates that former POWs frequently develop posttraumatic stress disorder and that for one half of those who develop the symptoms, they persist for over 40 years. Familial risk factors and preexisting psychopathology are superseded by the overwhelming nature of the trauma. The persistence of the symptoms for many years is a reflection of the severity of the trauma.