Thirty high-combat Vietnam veterans with a diagnosis of Post-traumatic Stress Disorder (PTSD) were compared with a second group of 30 high-combat Vietnam veterans without evidence of PTSD on measures of military adjustment and exposure to traumatic violence during combat. Military adjustment was assessed for precombat and combat service periods and included measures of alcohol and drug use, disciplinary actions, and social support. The PTSD-positive group reported significantly greater exposure to traumatic violence and more distress at having observed and participated in such acts than did the PTSD-negative group. Multiple regression analysis revealed the five traumatic violence frequency and distress scales to be significant predictors of severity of PTSD symptoms. Group differences were not attributable to premilitary demographic or social adjustment variables. Analyses of covariance demonstrated that both groups increased their drug and alcohol use and reported fewer social supports from precombat to combat periods. Results support the residual stress model of PTSD etiology, implicating trauma as the major contributing factor in the disorder.