This study ascertained the prevalence of posttraumatic stress disorder (PTSD) among chemically dependent adolescents and identified factors that influence the risk of PTSD after a qualifying trauma. Method: The study group consisted of 297 adolescents aged 15-19 years who met the DSM-III-R criteria for dependence on alcohol or other drugs and who were receiving treatment in seven publicly funded Massachusetts facilities. PTSD and other axis I diagnoses were assessed by the Diagnostic Interview Schedule. Data on risk factors were collected by a specially constructed interview schedule. Results: The lifetime prevalence of PTSD was 29.6% (24.3% for males and 45.3% for females), and the current prevalence was 19.2% (12.2% for males and 40.0% for females). These prevalences reflect a high occurrence of traumatic exposures and a high case rate among those who experienced trauma. The risk of PTSD varied with the nature of the trauma, the number of traumas experienced, psychiatric comorbidity, and familial characteristics. The higher rate of PTSD among females was due to a greater frequency of rape, which carries a high risk of PTSD development, and to a high rate of comorbid conditions. Conclusions: The lifetime prevalence of PTSD among these chemically dependent adolescents is five times that reported for a community sample of adolescents. This extremely high rate provides new understanding of the etiologic connection between PTSD and chemical dependence and has implications for their treatment.