We reviewed U.S. Army medical boards (136 cases) held between October 1990 and July 1994 for posttraumatic stress disorder (PTSD) that involved participation in the Persian Gulf War of 1990 to 1991. Thirty-five percent of these soldiers (34 cases) had also served in Vietnam. Their records were compared with the records of 102 other soldiers also medically retired for PTSD who served in the Persian Gulf War but did not serve in Vietnam. Approximately one-half of the Vietnam group developed PTSD symptoms in anticipation of deployment to the Persian Gulf. Those soldiers with prior Vietnam service had statistically significant odds ratios for PTSD (between about 5 and 24) compared with soldiers without Vietnam service. These findings indicate that for some persons with prior war experience, the threat of another war is sufficient to exacerbate symptoms or provoke a new episode of PTSD and this risk is substantially greater than that for soldiers without such experience.