Juvenile conduct disorder (CD) is a well-documented risk factor for posttraumatic stress disorder (PTSD). This study examines the mechanisms underlying this relationship by using data from 3,315 twin pairs in the Vietnam Era Twin Registry. Results indicate the number of conduct disorder symptoms increased risk of trauma exposure and PTSD in a dose–response fashion. This increased risk was mediated in part by the positive association between CD and lifestyle factors and was not due to confounding by shared genetic or familial vulnerability. The findings suggest CD increases risk for trauma exposure and PTSD among male veterans through direct and indirect mechanisms. Veterans who have a history of CD are at high risk for trauma exposure and development of PTSD.