This study explores relationships among childhood sexual abuse (CSA), age of substance use initiation, additional traumatic events, and posttraumatic stress disorder (PTSD) in a sample of adolescents. A history of CSA that preceded substance use was not related to an earlier age of substance use initiation. Early initiation of substance use predicted exposure to additional traumatic experiences. This relationship was partially mediated by engagement in risky behavior while under the influence of substances. Posttraumatic stress disorder was related to CSA, additional traumatic experiences and engagement in risky behavior while under the influence of substances.