The present study examined the association of childhood risk factors with exposure to traumas and posttraumatic stress disorder (PTSD). PTSD is a unique symptom configuration after exposure to an unusual, extreme event. Data come from the US National Comorbidity Study of 5,877 respondents aged 15-54 years conducted between September 1990 and February 1992. The risk factors examined were preexposure affective, anxiety, and substance use disorders; parental mental and substance use disorders; parental aggression toward the respondent and toward the other parent; and a nonconfiding relationship with the mother during childhood. Analyses were stratified by gender and adjusted for demographic variables and traumatic experiences prior to the index trauma. The occurrence of trauma was associated with many risk factors in women but few in men. Similarly, more risk factors predicted PTSD in women than in men. Overall, when respondents were grouped into broad trauma categories, an increase in the number of risk factors was associated with higher rates of PTSD. However, in analyses of the trauma subsample that adjusted for individual type of trauma (e.g., rape, physical attack), only one risk factor (history of affective disorder) predicted PTSD in women, and two (history of anxiety disorder and parental mental disorder) predicted PTSD in men. The results thus indicate that although these risk factors have an important association with PTSD, they operate largely by predicting trauma exposure rather than by predicting the onset of disorder after exposure. Am J Epidemiol 1998; 147: 353-61.