This study evaluates the impact of military service during the Vietnam era (1965 to 1975) on posttraumatic stress disorder using a sample of 2092 male-male, monozygotic, veteran twin pairs. Data were collected in 1987 using mail and telephone interviews. In 715 monozygotic twin pairs who were discordant for military service in southeast Asia (SEA), posttraumatic stress disorder was found to be strongly associated with military service in SEA. The prevalence of posttraumatic stress disorder was 16.8% in twins who served in SEA compared with 5.0% in co-twins who did not serve in SEA. There was a ninefold increase in the prevalence of posttraumatic stress disorder (95% confidence interval, 4.8 to 17.6), comparing twins who experienced high levels of combat with their co-twin who did not serve in SEA. Our results demonstrate that nearly 15 years following the end of the Vietnam War, there remains a substantially increased prevalence of posttraumatic stress disorder among veterans who served in SEA.